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

[Viktoriia Bakalova via llvm-commits]

VitaNuo wrote:

After some internal discussions, we have decided to put this work on hold.
It still remains an important problem. I have posted some stats for various open source projects in the [RFC](https://discourse.llvm.org/t/rfc-clang-diagnostic-for-demangling-failures/82835). Internally, we have around 177k violations (mangled names that can’t be demangled by the LLVM demangler).

In case someone in the community would like to pick up the effort, this PR has the code for the diagnostic. I will also [add a comment](https://github.com/llvm/llvm-project/pull/117346) to the codegen module mentioning the invariant that should hold in the future.

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