These headers have become very small by using compiler builtins, often
containing only two declarations. This merges these headers, since
there doesn't seem to be much of a benefit keeping them separate.
Specifically, `is_{,_nothrow,_trivially}{assignable,constructible}` are
kept and the `copy`, `move` and `default` versions of these type traits
are moved in to the respective headers.
According to https://developer.apple.com/support/xcode/, quite a few of
our availability macros don't do anything anymore, so we might as well
remove them to clean up the code a bit.
This is an implementation detail for `move_only_function` (and potentially other type-erasing classes).
Reviewed By: #libc, ldionne
Spies: Mordante, ldionne, EricWF, libcxx-commits
Differential Revision: https://reviews.llvm.org/D140259
