[clang][NFC] Refactor logic for picking standard library on Apple

Flip the logic around: always default to libc++ except on older platforms,
instead of defaulting to libstdc++ except on newer platforms. Since roughly
all supported platforms use libc++ now, it makes more sense to make that
the default, and allows the removal of some downstream diff.

Differential Revision: https://reviews.llvm.org/D122232