[CMake] Don't use CLANG_DEFAULT_* values
These are not necessarily correct, just because Clang is configured
to use libc++ or compiler-rt as defaults doesn't mean that the host
compiler uses these as defaults as well. A more correct solution
would be to test the host compiler to check what the default actually
is as attempted in D46857.
Part of PR-38025.
Differential Revision: https://reviews.llvm.org/D49272