darwin stdenv: Make stdenv.cc, not stdenv, bring in libcxx

stdenvNoCC should not inject any C++ standard library, just as it
doesn't inject any C standard library. stdenv still does, but only
indirectly through stdenv.cc. Wrapped clangs can be simplified now that
they don't need to worry about clobbering CoreFoundation when replacing
the C++ standard library implementation.

This generally-good cleanup should assist with debugging some C++
failures in #26805.

pkgs/development/compilers/llvm/5/default.nix

@@ -56,15 +56,11 @@ let
     stdenv = stdenv.override (drv: {
       allowedRequisites = null;
       cc = self.clang;
-      # Don't include the libc++ and libc++abi from the original stdenv.
-      extraBuildInputs = stdenv.lib.optional stdenv.isDarwin darwin.CF;
     });
 
     libcxxStdenv = stdenv.override (drv: {
       allowedRequisites = null;
       cc = self.libcxxClang;
-      # Don't include the libc++ and libc++abi from the original stdenv.
-      extraBuildInputs = stdenv.lib.optional stdenv.isDarwin darwin.CF;
     });
 
     lld = callPackage ./lld.nix {};