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

[Erich Keane 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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erichkeane wrote:

```suggestion
def fdiag-demangler_failures: Flag<["-"], "fdiag-demangler-failures">, Group<f_clang_Group>,
```
Same as above... I'm not attached to the demangling vs demangler, but mentioning explicitly that this is for 'diag' everywhere is necessary for readability.

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