[libc++] Always define a key function for std::bad_function_call in the dylib

However, whether applications rely on the std::bad_function_call vtable
being in the dylib is still controlled by the ABI macro, since changing
that would be an ABI break.

Also separate preprocessor definitions for whether to use a key function
and whether to use a `bad_function_call`-specific `what` message
(`what` message is mandated by [LWG2233](http://wg21.link/LWG2233)).

Differential Revision: https://reviews.llvm.org/D92397