[libc++] Use availability to rely on key functions for bad_expected_access and...

[libc++] Use availability to rely on key functions for bad_expected_access and bad_function_call (#87390)

This patch uses our availability machinery to allow defining a key
function for bad_function_call and bad_expected_access at all times but
only rely on it when we can. This prevents compilers from complaining
about weak vtables and reduces code bloat and the amount of work done by
the dynamic linker.

rdar://111917845