[clang] [Clang] Add [[clang::complete_on_member_instantiation]] (PR #211231)

[Louis Dionne via cfe-commits]

https://github.com/ldionne commented:

I'm mostly neutral on this. Objectively, this undeniably improves the diagnostics and simplifies the library over what we can do with a pure library approach (see https://github.com/llvm/llvm-project/pull/211110). On the other hand, this is a very specific attribute, so I would understand if Clang doesn't want to add it.

If we do move forward, I have a few comments / suggestions.

https://github.com/llvm/llvm-project/pull/211231