llvmPackages.clang: move add-nostdlibinc-flag.patch to cc-wrapper

pkgs/build-support/cc-wrapper/default.nix

@@ -629,6 +629,20 @@ stdenvNoCC.mkDerivation {
       echo " -L${libcxx_solib}" >> $out/nix-support/cc-ldflags
     ''
 
+    ## Prevent clang from seeing /usr/include. There is a desire to achieve this
+    ## through alternate means because it breaks -sysroot and related functionality.
+    #
+    # This flag prevents global system header directories from
+    # leaking through on non‐NixOS Linux. However, on macOS, the
+    # SDK path is used as the sysroot, and forcing `-nostdlibinc`
+    # breaks `-isysroot` with an unwrapped compiler. As macOS has
+    # no `/usr/include`, there’s essentially no risk to dropping
+    # the flag there. See discussion in NixOS/nixpkgs#191152.
+    #
+    + optionalString ((cc.isClang or false) && !targetPlatform.isDarwin) ''
+      echo " -nostdlibinc" >> $out/nix-support/cc-cflags
+    ''
+
     ##
     ## Man page and info support
     ##

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

@@ -529,23 +529,6 @@ let
             # mis-compilation in firefox.
             # See: https://bugzilla.mozilla.org/show_bug.cgi?id=1741454
             (metadata.getVersionFile "clang/revert-malloc-alignment-assumption.patch")
-          # This patch prevents global system header directories from
-          # leaking through on non‐NixOS Linux. However, on macOS, the
-          # SDK path is used as the sysroot, and forcing `-nostdlibinc`
-          # breaks `-isysroot` with an unwrapped compiler. As macOS has
-          # no `/usr/include`, there’s essentially no risk to skipping
-          # the patch there. It’s possible that Homebrew headers in
-          # `/usr/local/include` might leak through to unwrapped
-          # compilers being used without an SDK set or something, but
-          # it hopefully shouldn’t matter.
-          #
-          # TODO: Figure out a better solution to this whole problem so
-          # that we won’t have to choose between breaking unwrapped
-          # compilers breaking libclang when we can do Linux‐to‐Darwin
-          # cross‐compilation again.
-          ++ lib.optional (
-            !args.stdenv.hostPlatform.isDarwin || !args.stdenv.targetPlatform.isDarwin
-          ) ./clang/add-nostdlibinc-flag.patch
           ++ [
             (substituteAll {
               src =