Add macOS libs ahead of brewed llvm libs in lib search path

This ensures that libraries that are built with brewed LLVM but not
included in the Command Line Tools/Xcode (e.g. libomp) can be found
during a build, while still using system libraries for the essential
stuff (e.g. libc++)

Library/Homebrew/extend/ENV/super.rb

@@ -176,6 +176,8 @@ module Superenv
     PATH.new(
       keg_only_deps.map(&:opt_lib),
       HOMEBREW_PREFIX/"lib",
+      "#{MacOS.sdk_path}/usr/lib",
+      (compiler == :llvm_clang ? Formula["llvm"].opt_lib.to_s : ""),
       homebrew_extra_library_paths,
     ).existing
   end