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

[Viktoriia Bakalova via cfe-commits]

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@@ -1967,6 +1967,10 @@ def fclang_abi_compat_EQ : Joined<["-"], "fclang-abi-compat=">, Group<f_clang_Gr
   Visibility<[ClangOption, CC1Option]>,
   MetaVarName<"<version>">, Values<"<major>.<minor>,latest">,
   HelpText<"Attempt to match the ABI of Clang <version>">;
+def fdemangling_failures: Flag<["-"], "fdemangling-failures">, Group<f_clang_Group>,
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VitaNuo wrote:

Thank you, good point. I've looked through the existing flags, there are multiple ones that start with `fdiagnostics_XXX`, so I have renamed the flag to `fdiagnostics_demangler_failures`. 
Let me know if this works.

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