[clang] [clang] Implement -Walloc-size diagnostic option (PR #150028)

[Erich Keane via cfe-commits]

https://github.com/erichkeane commented:

1 have 1 concern with how we're getting the value of alloc-size, and think we probably need a little tweaking, else this is a LGTM.

https://github.com/llvm/llvm-project/pull/150028