[clang] [clang][SME] Ignore flatten for callees with mismatched streaming attributes (PR #116391)

[Benjamin Maxwell via cfe-commits]

MacDue wrote:

> I guess this is fine, then. But please add a note describing the current situation, so the next person looking at this has context. (Put it in TargetInfo.h, maybe?)
> 
> LGTM with the note.

Thanks :+1:  Added a note to `TargetInfo.h` 

https://github.com/llvm/llvm-project/pull/116391