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.
Package changes from 3.6:
- CMake exports patch no longer necessary
- Cosmetic purity patch fix
- Build libc++ with private libc++abi headers visible from sources
- Work around bugs in lldb's configure scripts
