[clang/Diagnostic] Use `optional` to disambiguate between a...

[clang/Diagnostic] Use `optional` to disambiguate between a `StoredDiagMessage` that is not set vs set as empty string

"But when would you have a completely empty diagnostic message", you ask dear reader?
That is when there is an empty "#warning" in code.

rdar://106155415

Differential Revision: https://reviews.llvm.org/D145256