[clang] [llvm] [clang][LLVM Demangler] Add a diagnostic that validates that all mang… (PR #111391)

[Viktoriia Bakalova via cfe-commits]

VitaNuo wrote:

> Close for me, jsut a couple of small requests.
> 
> Thanks for your patience on this response, I've been at LLVMDev all week.

Thanks Erich for the approval. 
I will publish an RFC for this in the coming days as well, still gathering some data from test runs.

https://github.com/llvm/llvm-project/pull/111391