[Clang] Implement P2169 A nice placeholder with no name

This is a C++ feature that allows the use of `_` to
declare multiple variable of that name in the same scope;
these variables can then not be referred to.

In addition, while P2169 does not extend to parameter
declarations, we stop warning on unused parameters of that name,
for consistency.

The feature is backported to all C++ language modes.

Reviewed By: #clang-language-wg, aaron.ballman

Differential Revision: https://reviews.llvm.org/D153536