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

[Erich Keane via cfe-commits]

erichkeane wrote:

> The RFC is published https://discourse.llvm.org/t/rfc-clang-diagnostic-for-demangling-failures/82835.

Thanks!  I thought i commented elsewhere, but I think `diagnose` vs `diagnostics` for the flags/etc makes more sense:
`fdiagnostics_demangler_failures` should be `-fdiagnose_demangler_failures`, it is the correct language-tense.

Else, the patch is good delta what happens with the RFC.  Please make sure to wait until Aaron calls the RFC.

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