[libc++] Use __is_convertible built-in when available

https://github.com/llvm/llvm-project/issues/62396 reports that
GCC 13 barfs on parsing <type_traits> because of the declarations
of `struct __is_convertible`.  In GCC 13, `__is_convertible` is a
built-in, but `__is_convertible_to` is not.  Clang has both, so
using either should be fine.

Reviewed By: #libc, philnik

Differential Revision: https://reviews.llvm.org/D149313