[clang] Improve the -Wundefined-func-template diagnostic note for invisible template functions (PR #129031)

[Haojian Wu via cfe-commits]

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@@ -5082,11 +5085,18 @@ void Sema::InstantiateFunctionDefinition(SourceLocation PointOfInstantiation,
       if (AtEndOfTU && !getDiagnostics().hasErrorOccurred() &&
           !getSourceManager().isInSystemHeader(PatternDecl->getBeginLoc())) {
         Diag(PointOfInstantiation, diag::warn_func_template_missing)
-          << Function;
-        Diag(PatternDecl->getLocation(), diag::note_forward_template_decl);
-        if (getLangOpts().CPlusPlus11)
-          Diag(PointOfInstantiation, diag::note_inst_declaration_hint)
-              << Function;
+            << Function;
+        if (Unreachable) {
+          // FIXME: would be nice to mention which module the function template
+          // comes from. 
+          Diag(PatternDecl->getLocation(),
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hokein wrote:

The `note_inst_declaration_hint`  is about a fix hint which suggests to add an explicit instantiation declaration to suppress this warning, it is not feasible to the unreachable-template-decl case.

https://github.com/llvm/llvm-project/pull/129031