[clang] Add fallback type for Constrained placeholders other than plain auto. (PR #209893)

[Erich Keane via cfe-commits]

erichkeane wrote:

> @erichkeane very minor nit here: the bullet under "Bug fixes to C++ support" is not spaced and doesn't render properly.

Thanks for the heads up!  I TOLD @aaronballman that I'd screw this up being the first one in here :D 

https://github.com/llvm/llvm-project/pull/209893