llvmPackages.{clang,mlir}: Mark big-parallel

Mark these packages as big-parallel to match llvmPackages.llvm.

They are chosen because they are relatively expensive to build, as
opposed to others where I currently believe they are less of an issue
from a CPU time/memory standpoint.

I suspect most people don't care about big-parallel or it would have
been noticed earlier. I have a use case where I am building many
variants of clang all at once and I want to avoid OOM by constraining
the number of parallel builds of items marked big-parallel.

Signed-off-by: Peter Waller <p@pwaller.net>

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

@@ -298,6 +298,7 @@ stdenv.mkDerivation (
         ++ (finalAttrs.passthru.hardeningUnsupportedFlags or [ ]);
     };
 
+    requiredSystemFeatures = [ "big-parallel" ];
     meta = llvm_meta // {
       homepage = "https://clang.llvm.org/";
       description = "C language family frontend for LLVM";

pkgs/development/compilers/llvm/common/mlir/default.nix

@@ -87,6 +87,7 @@ stdenv.mkDerivation (finalAttrs: {
     "dev"
   ];
 
+  requiredSystemFeatures = [ "big-parallel" ];
   meta = llvm_meta // {
     # Very broken since the dependencies aren't propagating at all with tblgen through the CMake.
     broken = lib.versionAtLeast release_version "20";