[clang][NFC] Add a notice to desugarForDiagnostic

`desugarForDiagnostic` only sets ShouldAKA to true if desugaring
happens, otherwise ShouldAKA is left intact and might be uninitialized.

Victims (including me):

https://github.com/llvm/llvm-project/commit/25bf8cb3c0e3c41231289a6ff0a37b6d49b24011

https://github.com/llvm/llvm-project/commit/0e8384a0fe4f03d60cd92aba1cae074512481ca2

Reviewed By: aaron.ballman

Differential Revision: https://reviews.llvm.org/D152880