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.

(cherry picked from commit 37437849de)

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

@@ -78,17 +78,7 @@ let
       substituteInPlace lib/Driver/ToolChains/Darwin.cpp \
         --replace-fail 'StringRef P = llvm::sys::path::parent_path(D.Dir);' 'StringRef P = "${lib.getLib libllvm}";'
       (cd tools && ln -s ../../clang-tools-extra extra)
-    '' + lib.optionalString (
-      lib.versionOlder release_version "13"
-      # See the comment on the `add-nostdlibinc-flag.patch` patch in
-      # `../default.nix` for why we skip Darwin here.
-      && (!stdenv.hostPlatform.isDarwin || !stdenv.targetPlatform.isDarwin
-    ) ''
-      sed -i -e 's/DriverArgs.hasArg(options::OPT_nostdlibinc)/true/' \
-             -e 's/Args.hasArg(options::OPT_nostdlibinc)/true/' \
-             lib/Driver/ToolChains/*.cpp
-    ''
-    + lib.optionalString (lib.versionOlder release_version "13") ''
+    '' + lib.optionalString (lib.versionOlder release_version "13") ''
       substituteInPlace tools/extra/clangd/quality/CompletionModel.cmake \
         --replace ' ''${CMAKE_SOURCE_DIR}/../clang-tools-extra' ' ''${CMAKE_SOURCE_DIR}/tools/extra'
     ''

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

@@ -555,9 +555,7 @@ let
           # compilers breaking libclang when we can do Linux‐to‐Darwin
           # cross‐compilation again.
           ++ lib.optional (
-            # TODO: This also applies for clang == 12, do we need it?
-            lib.versionAtLeast metadata.release_version "13" &&
-            (!args.stdenv.hostPlatform.isDarwin || !args.stdenv.targetPlatform.isDarwin)
+            !args.stdenv.hostPlatform.isDarwin || !args.stdenv.targetPlatform.isDarwin
           ) ./clang/add-nostdlibinc-flag.patch
           ++ [
             (substituteAll {