[clang] [BoundsSafety][NFC] Introduce logic to replace the LateParsedAttrType placeholder with CountAttributedType (PR #212906)

[Erich Keane via cfe-commits]

erichkeane wrote:

@cor3ntin actually pointed out privately that we actually already have something EXACTLY like this functionality that does exactly what you want it to, that would be pretty trivially modeled after: Inline C++ functions.  See `ParseLexedMethodDefs`.  I think we should be trivially able to add that sort of functionality for this, AND it would work in a way that we've spent decades proving does the 'right' thing.  IMO, this is just the 'right' way to do it.

https://github.com/llvm/llvm-project/pull/212906