haskell.compiler.*: only set CLANG to match llc/opt

* By taking clang from llvmPackages we make sure there is no version
  mismatch between LLVM (where llc and opt come from) and clang (which
  previously would be taken from stdenv on darwin for example).

* Only pass CLANG if useLLVM is true. Previously we also set this if
  targetCC was clang. This would cause potentially confusing behavior if
  llc and opt as well as clang are provided via PATH (and GHC is
  compiled with useLLVM == false), because clang from PATH would be
  ignored, but not llc and opt.

pkgs/development/compilers/ghc/8.10.7.nix

@@ -214,13 +214,10 @@ stdenv.mkDerivation (rec {
   '' + lib.optionalString useLLVM ''
     export LLC="${lib.getBin llvmPackages.llvm}/bin/llc"
     export OPT="${lib.getBin llvmPackages.llvm}/bin/opt"
-  '' + lib.optionalString (targetCC.isClang || (useLLVM && stdenv.targetPlatform.isDarwin)) (let
-    # LLVM backend on Darwin needs clang, if we are already using clang, might as well set the environment variable.
-    # See also https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/codegens.html#llvm-code-generator-fllvm
-    clang = if targetCC.isClang then targetCC else llvmPackages.clang;
-  in ''
-    export CLANG="${clang}/bin/${clang.targetPrefix}clang"
-  '') + ''
+  '' + lib.optionalString (useLLVM && stdenv.targetPlatform.isDarwin) ''
+    # LLVM backend on Darwin needs clang: https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/codegens.html#llvm-code-generator-fllvm
+    export CLANG="${llvmPackages.clang}/bin/${llvmPackages.clang.targetPrefix}clang"
+  '' + ''
 
     echo -n "${buildMK}" > mk/build.mk
     sed -i -e 's|-isysroot /Developer/SDKs/MacOSX10.5.sdk||' configure

pkgs/development/compilers/ghc/8.8.4.nix

@@ -219,13 +219,10 @@ stdenv.mkDerivation (rec {
   '' + lib.optionalString useLLVM ''
     export LLC="${lib.getBin llvmPackages.llvm}/bin/llc"
     export OPT="${lib.getBin llvmPackages.llvm}/bin/opt"
-  '' + lib.optionalString (targetCC.isClang || (useLLVM && stdenv.targetPlatform.isDarwin)) (let
-    # LLVM backend on Darwin needs clang, if we are already using clang, might as well set the environment variable.
-    # See also https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/codegens.html#llvm-code-generator-fllvm
-    clang = if targetCC.isClang then targetCC else llvmPackages.clang;
-  in ''
-    export CLANG="${clang}/bin/${clang.targetPrefix}clang"
-  '') + ''
+  '' + lib.optionalString (useLLVM && stdenv.targetPlatform.isDarwin) ''
+    # LLVM backend on Darwin needs clang: https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/codegens.html#llvm-code-generator-fllvm
+    export CLANG="${llvmPackages.clang}/bin/${llvmPackages.clang.targetPrefix}clang"
+  '' + ''
 
     echo -n "${buildMK dontStrip}" > mk/build.mk
     sed -i -e 's|-isysroot /Developer/SDKs/MacOSX10.5.sdk||' configure

pkgs/development/compilers/ghc/9.0.1.nix

@@ -180,13 +180,10 @@ stdenv.mkDerivation (rec {
   '' + lib.optionalString useLLVM ''
     export LLC="${lib.getBin llvmPackages.llvm}/bin/llc"
     export OPT="${lib.getBin llvmPackages.llvm}/bin/opt"
-  '' + lib.optionalString (targetCC.isClang || (useLLVM && stdenv.targetPlatform.isDarwin)) (let
-    # LLVM backend on Darwin needs clang, if we are already using clang, might as well set the environment variable.
-    # See also https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/codegens.html#llvm-code-generator-fllvm
-    clang = if targetCC.isClang then targetCC else llvmPackages.clang;
-  in ''
-    export CLANG="${clang}/bin/${clang.targetPrefix}clang"
-  '') + ''
+  '' + lib.optionalString (useLLVM && stdenv.targetPlatform.isDarwin) ''
+    # LLVM backend on Darwin needs clang: https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/codegens.html#llvm-code-generator-fllvm
+    export CLANG="${llvmPackages.clang}/bin/${llvmPackages.clang.targetPrefix}clang"
+  '' + ''
 
     echo -n "${buildMK}" > mk/build.mk
     sed -i -e 's|-isysroot /Developer/SDKs/MacOSX10.5.sdk||' configure

pkgs/development/compilers/ghc/9.2.1.nix

@@ -180,13 +180,10 @@ stdenv.mkDerivation (rec {
   '' + lib.optionalString useLLVM ''
     export LLC="${lib.getBin llvmPackages.llvm}/bin/llc"
     export OPT="${lib.getBin llvmPackages.llvm}/bin/opt"
-  '' + lib.optionalString (targetCC.isClang || (useLLVM && stdenv.targetPlatform.isDarwin)) (let
-    # LLVM backend on Darwin needs clang, if we are already using clang, might as well set the environment variable.
-    # See also https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/codegens.html#llvm-code-generator-fllvm
-    clang = if targetCC.isClang then targetCC else llvmPackages.clang;
-  in ''
-    export CLANG="${clang}/bin/${clang.targetPrefix}clang"
-  '') + ''
+  '' + lib.optionalString (useLLVM && stdenv.targetPlatform.isDarwin) ''
+    # LLVM backend on Darwin needs clang: https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/codegens.html#llvm-code-generator-fllvm
+    export CLANG="${llvmPackages.clang}/bin/${llvmPackages.clang.targetPrefix}clang"
+  '' + ''
 
     echo -n "${buildMK}" > mk/build.mk
     sed -i -e 's|-isysroot /Developer/SDKs/MacOSX10.5.sdk||' configure

pkgs/development/compilers/ghc/head.nix

@@ -194,13 +194,10 @@ stdenv.mkDerivation (rec {
   '' + lib.optionalString useLLVM ''
     export LLC="${lib.getBin llvmPackages.llvm}/bin/llc"
     export OPT="${lib.getBin llvmPackages.llvm}/bin/opt"
-  '' + lib.optionalString (targetCC.isClang || (useLLVM && stdenv.targetPlatform.isDarwin)) (let
-    # LLVM backend on Darwin needs clang, if we are already using clang, might as well set the environment variable.
-    # See also https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/codegens.html#llvm-code-generator-fllvm
-    clang = if targetCC.isClang then targetCC else llvmPackages.clang;
-  in ''
-    export CLANG="${clang}/bin/${clang.targetPrefix}clang"
-  '') + ''
+  '' + lib.optionalString (useLLVM && stdenv.targetPlatform.isDarwin) ''
+    # LLVM backend on Darwin needs clang: https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/codegens.html#llvm-code-generator-fllvm
+    export CLANG="${llvmPackages.clang}/bin/${llvmPackages.clang.targetPrefix}clang"
+  '' + ''
 
     # otherwise haddock fails when generating the compiler docs
     export LANG=C.UTF-8