[clang] [clang][Sema] Combine fallout warnings to just one warning (PR #127546)

[via cfe-commits]

foxtran wrote:

Looks like I cannot have some variable with proper type generated from `%enum_select`.

So, for
```
"%enum_select<FunModes>{%Function{function}|%Block{block}|%Lambda{lambda}|%Coroutine{coroutine}}0"
```
I can not find corresponding type to save enum values into specific container (expected `diag::FunModes`). 

https://github.com/llvm/llvm-project/pull/127546