[clang] [clang-tools-extra] [Clang] [C2y] Implement N3355 ‘NamedLoops’ (PR #152870)

[via cfe-commits]

Sirraide wrote:

> We are quite early in the C2y cycle, there have been proposals altering the design, and C++ has express interest in doing something maybe similar.
> We should gather deployment experience before backing us, and committees in a corner.

Sure; I just wanted to make sure this implementation also works for C++ because I wanted to avoid having to refactor this if/when C++ also gets this as a feature.

https://github.com/llvm/llvm-project/pull/152870