[clang] [clang][BoundsSafety] Bring nested struct support into AST/Sema (PR #186226)

[John Paul Jepko via cfe-commits]

jpjepko wrote:

Closing after discussion with Yeoul. Parts of this patch may be useful in the future, but as of now it's likely bringing in too many things at once.

https://github.com/llvm/llvm-project/pull/186226