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

[Erich Keane via cfe-commits]

================
@@ -384,4 +384,8 @@ def warn_hlsl_langstd_minimal :
   Warning<"support for HLSL language version %0 is incomplete, "
           "recommend using %1 instead">,
   InGroup<HLSLDXCCompat>;
+
+def warn_name_cannot_be_demangled : Warning<
+  "cannot demangle the name '%0'">,
+  InGroup<CXX20Compat>;
----------------
erichkeane wrote:

Group is wrong here, we likely want to give it its own group for the diagnostic.  

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