llvmPackages: Add an update script

This is based on a combination of Gabor Greif's update script [0] and my
script to verify the hashes [1]. It's a bit hacky / unconventional but
it should be very useful and worked fine so far (but it might require
changes for some edge-cases).

[0]: https://github.com/NixOS/nixpkgs/issues/100725#issuecomment-710055349
[1]: https://github.com/NixOS/nixpkgs/issues/100725#issuecomment-781323979

pkgs/development/compilers/llvm/update.sh

@@ -0,0 +1,44 @@
+#!/usr/bin/env bash
+
+set -eu
+
+# Example usage: ./pkgs/development/compilers/llvm/update.sh 12.0.0-rc4
+
+readonly VERSION="$1"
+readonly VERSION_MAJOR="${VERSION%%.*}"
+readonly VERSION_MAIN="${VERSION%%-*}"
+declare VERSION_PATCH="${VERSION/$VERSION_MAIN/}"
+readonly VERSION_PATCH="${VERSION_PATCH/-/}"
+
+readonly DIR="pkgs/development/compilers/llvm/$VERSION_MAJOR"
+readonly FILE="$DIR/default.nix"
+
+sed -Ei \
+  -e "s/release_version = \".+\";/release_version = \"$VERSION_MAIN\";/" \
+  -e "s/candidate = \".*\";/candidate = \"$VERSION_PATCH\";/" \
+  "$FILE"
+
+readonly ATTRSET="llvmPackages_$VERSION_MAJOR"
+readonly SOURCES=(
+  "clang-unwrapped.src"
+  "compiler-rt.src"
+  "clang-unwrapped.clang-tools-extra_src"
+  "libcxx.src"
+  "libcxxabi.src"
+  "libunwind.src"
+  "lld.src"
+  "lldb.src"
+  "llvm.src"
+  "llvm.polly_src"
+  "openmp.src"
+)
+
+for SOURCE in "${SOURCES[@]}"; do
+  echo "Updating the hash of $SOURCE:"
+  declare ATTR="$ATTRSET.$SOURCE"
+  declare OLD_HASH="$(nix eval -f . $ATTR.outputHash)"
+  declare NEW_HASH="\"$(nix-prefetch-url -A $ATTR)\""
+  find "$DIR" -type f -exec sed -i "s/$OLD_HASH/$NEW_HASH/" {} +
+done
+
+echo OK