[libc++] Simplify __config a bit

Simplify logic in `__config` by assuming that we are using Clang in C++03 mode. Also, use standardized feature-test macros instead of compiler-specific checks (like `__has_feature`) in a couple of places.

Reviewed By: ldionne, #libc

Spies: libcxx-commits

Differential Revision: https://reviews.llvm.org/D127606