[libc++] Properly handle specializations of std::is_placeholder.

Before this patch, the user needed to specialize both of
`is_placeholder<MyType>` and `is_placeholder<const MyType>`.
After this patch, only the former is needed (although the
latter is harmless if provided).

The new tests don't actually fail unless return type deduction
is used, which is a C++14 feature. Specializing `is_placeholder`
is still allowed in C++11, though.

Fixes #51095.

Differential Revision: https://reviews.llvm.org/D116388