llvmPackages_12.clang: use nostdlibinc patch instead of sed command

Seems like the patch we've written for LLVM 13 and above also works for
LLVM 12 which seems a little more robust.

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

@@ -81,10 +81,6 @@ let
         --replace-fail 'StringRef P = llvm::sys::path::parent_path(D.Dir);' 'StringRef P = "${lib.getLib libllvm}";'
       (cd tools && ln -s ../../clang-tools-extra extra)
     '' + lib.optionalString (lib.versionOlder release_version "13") ''
-      sed -i -e 's/DriverArgs.hasArg(options::OPT_nostdlibinc)/true/' \
-             -e 's/Args.hasArg(options::OPT_nostdlibinc)/true/' \
-             lib/Driver/ToolChains/*.cpp
-
       substituteInPlace tools/extra/clangd/quality/CompletionModel.cmake \
         --replace ' ''${CMAKE_SOURCE_DIR}/../clang-tools-extra' ' ''${CMAKE_SOURCE_DIR}/tools/extra'
     ''

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

@@ -540,11 +540,9 @@ let
             # mis-compilation in firefox.
             # See: https://bugzilla.mozilla.org/show_bug.cgi?id=1741454
             (metadata.getVersionFile "clang/revert-malloc-alignment-assumption.patch")
-          ++ lib.optionals (lib.versionAtLeast metadata.release_version "13") [
-            # TODO: This also applies for clang == 12, do we need it?
-            ./clang/add-nostdlibinc-flag.patch
-          ]
           ++ [
+            ./clang/add-nostdlibinc-flag.patch
+
             (substituteAll {
               src =
                 if (lib.versionOlder metadata.release_version "16") then