llvmPackages_14.clang: include clang-tools-extra in src for use

Fixes #166833.

The build creates a symlink for this assuming it's present,
so be sure it's there when filtering the source for clang.

Alternatively we could use LLVM_EXTERNAL_CLANG_TOOLS_EXTRA_SOURCE_DIR.

pkgs/development/compilers/llvm/14/clang/default.nix

@@ -15,6 +15,7 @@ let
       mkdir -p "$out"
       cp -r ${monorepoSrc}/cmake "$out"
       cp -r ${monorepoSrc}/${pname} "$out"
+      cp -r ${monorepoSrc}/clang-tools-extra "$out"
     '';
 
     sourceRoot = "${src.name}/${pname}";