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

[Viktoriia Bakalova via cfe-commits]

VitaNuo wrote:

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


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