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.