llvmPackages.clang: dedupe purity.patch 5-8

found with fdupes

```
pkgs/development/compilers/llvm/8/clang/purity.patch
pkgs/development/compilers/llvm/5/clang/purity.patch
pkgs/development/compilers/llvm/6/clang/purity.patch
pkgs/development/compilers/llvm/7/clang/purity.patch
```

pkgs/development/compilers/llvm/5/clang/default.nix

@@ -41,7 +41,7 @@ let
     ];
 
     patches = [
-      ./purity.patch
+      ../../common/clang/5-8-purity.patch
       ./gnu-install-dirs.patch
       (substituteAll {
         src = ./LLVMgold-path.patch;

pkgs/development/compilers/llvm/6/clang/default.nix

@@ -41,7 +41,7 @@ let
     ];
 
     patches = [
-      ./purity.patch
+      ../../common/clang/5-8-purity.patch
       ./gnu-install-dirs.patch
       (substituteAll {
         src = ../../clang-6-10-LLVMgold-path.patch;

pkgs/development/compilers/llvm/6/clang/purity.patch

@@ -1,30 +0,0 @@
-From 4add81bba40dcec62c4ea4481be8e35ac53e89d8 Mon Sep 17 00:00:00 2001
-From: Will Dietz <w@wdtz.org>
-Date: Thu, 18 May 2017 11:56:12 -0500
-Subject: [PATCH] "purity" patch for 5.0
-
----
- lib/Driver/ToolChains/Gnu.cpp | 7 -------
- 1 file changed, 7 deletions(-)
-
-diff --git a/lib/Driver/ToolChains/Gnu.cpp b/lib/Driver/ToolChains/Gnu.cpp
-index fe3c0191bb..c6a482bece 100644
---- a/lib/Driver/ToolChains/Gnu.cpp
-+++ b/lib/Driver/ToolChains/Gnu.cpp
-@@ -494,13 +494,6 @@ void tools::gnutools::Linker::ConstructJob(Compilation &C, const JobAction &JA,
-   if (!Args.hasArg(options::OPT_static)) {
-     if (Args.hasArg(options::OPT_rdynamic))
-       CmdArgs.push_back("-export-dynamic");
--
--    if (!Args.hasArg(options::OPT_shared)) {
--      const std::string Loader =
--          D.DyldPrefix + ToolChain.getDynamicLinker(Args);
--      CmdArgs.push_back("-dynamic-linker");
--      CmdArgs.push_back(Args.MakeArgString(Loader));
--    }
-   }
- 
-   CmdArgs.push_back("-o");
--- 
-2.11.0
-

pkgs/development/compilers/llvm/7/clang/default.nix

@@ -45,7 +45,7 @@ let
     ];
 
     patches = [
-      ./purity.patch
+      ../../common/clang/5-8-purity.patch
       # make clang -xhip use $PATH to find executables
       ./HIP-use-PATH-7.patch
       # Backport for the `--unwindlib=[libgcc|compiler-rt]` flag, which is

pkgs/development/compilers/llvm/7/clang/purity.patch

@@ -1,30 +0,0 @@
-From 4add81bba40dcec62c4ea4481be8e35ac53e89d8 Mon Sep 17 00:00:00 2001
-From: Will Dietz <w@wdtz.org>
-Date: Thu, 18 May 2017 11:56:12 -0500
-Subject: [PATCH] "purity" patch for 5.0
-
----
- lib/Driver/ToolChains/Gnu.cpp | 7 -------
- 1 file changed, 7 deletions(-)
-
-diff --git a/lib/Driver/ToolChains/Gnu.cpp b/lib/Driver/ToolChains/Gnu.cpp
-index fe3c0191bb..c6a482bece 100644
---- a/lib/Driver/ToolChains/Gnu.cpp
-+++ b/lib/Driver/ToolChains/Gnu.cpp
-@@ -494,13 +494,6 @@ void tools::gnutools::Linker::ConstructJob(Compilation &C, const JobAction &JA,
-   if (!Args.hasArg(options::OPT_static)) {
-     if (Args.hasArg(options::OPT_rdynamic))
-       CmdArgs.push_back("-export-dynamic");
--
--    if (!Args.hasArg(options::OPT_shared)) {
--      const std::string Loader =
--          D.DyldPrefix + ToolChain.getDynamicLinker(Args);
--      CmdArgs.push_back("-dynamic-linker");
--      CmdArgs.push_back(Args.MakeArgString(Loader));
--    }
-   }
- 
-   CmdArgs.push_back("-o");
--- 
-2.11.0
-

pkgs/development/compilers/llvm/8/clang/default.nix

@@ -45,7 +45,7 @@ let
     ];
 
     patches = [
-      ./purity.patch
+      ../../common/clang/5-8-purity.patch
       ./xpc.patch
       # Backport for -static-pie, which the latter touches, and which is nice in
       # its own right.

pkgs/development/compilers/llvm/8/clang/purity.patch

@@ -1,30 +0,0 @@
-From 4add81bba40dcec62c4ea4481be8e35ac53e89d8 Mon Sep 17 00:00:00 2001
-From: Will Dietz <w@wdtz.org>
-Date: Thu, 18 May 2017 11:56:12 -0500
-Subject: [PATCH] "purity" patch for 5.0
-
----
- lib/Driver/ToolChains/Gnu.cpp | 7 -------
- 1 file changed, 7 deletions(-)
-
-diff --git a/lib/Driver/ToolChains/Gnu.cpp b/lib/Driver/ToolChains/Gnu.cpp
-index fe3c0191bb..c6a482bece 100644
---- a/lib/Driver/ToolChains/Gnu.cpp
-+++ b/lib/Driver/ToolChains/Gnu.cpp
-@@ -494,13 +494,6 @@ void tools::gnutools::Linker::ConstructJob(Compilation &C, const JobAction &JA,
-   if (!Args.hasArg(options::OPT_static)) {
-     if (Args.hasArg(options::OPT_rdynamic))
-       CmdArgs.push_back("-export-dynamic");
--
--    if (!Args.hasArg(options::OPT_shared)) {
--      const std::string Loader =
--          D.DyldPrefix + ToolChain.getDynamicLinker(Args);
--      CmdArgs.push_back("-dynamic-linker");
--      CmdArgs.push_back(Args.MakeArgString(Loader));
--    }
-   }
- 
-   CmdArgs.push_back("-o");
--- 
-2.11.0
-