[clang] Adds a pseudonym to clang"s windows mangler... (PR #97792)

[Reid Kleckner via cfe-commits]

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@@ -1938,12 +1946,23 @@ void MicrosoftCXXNameMangler::mangleTemplateArgValue(QualType T,
         mangleNumber(V.getLValueOffset().getQuantity());
       } else if (!V.hasLValuePath()) {
         // FIXME: This can only happen as an extension. Invent a mangling.
-        break;
+        DiagnosticsEngine &Diags = Context.getDiags();
+        unsigned DiagID =
+            Diags.getCustomDiagID(DiagnosticsEngine::Error,
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rnk wrote:

Thank you for diagnosing these various errors, but can you please refactor the diagnostic emission out, similar to how it is done in `CodeGenModule::ErrorUnsupported`?
https://github.com/llvm/llvm-project/blob/main/clang/lib/CodeGen/CodeGenModule.cpp#L1549

https://github.com/llvm/llvm-project/pull/97792