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

[Shafik Yaghmour via cfe-commits]

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@@ -759,13 +759,11 @@ Sema::BuildDependentDeclRefExpr(const CXXScopeSpec &SS,
       TemplateArgs);
 }
 
-bool Sema::DiagnoseUninstantiableTemplate(SourceLocation PointOfInstantiation,
-                                          NamedDecl *Instantiation,
-                                          bool InstantiatedFromMember,
-                                          const NamedDecl *Pattern,
-                                          const NamedDecl *PatternDef,
-                                          TemplateSpecializationKind TSK,
-                                          bool Complain /*= true*/) {
+bool Sema::DiagnoseUninstantiableTemplate(
+    SourceLocation PointOfInstantiation, NamedDecl *Instantiation,
+    bool InstantiatedFromMember, const NamedDecl *Pattern,
+    const NamedDecl *PatternDef, TemplateSpecializationKind TSK,
+    bool Complain /*= true*/, bool *Unreachable) {
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shafik wrote:

@AaronBallman I don't think I have seen annotation before, annotating the default value in a function definition.

Are there tools that verify this like we have for [argument comments?](https://clang.llvm.org/extra/clang-tidy/checks/bugprone/argument-comment.html) 

Otherwise I don't think these make as much sense, although I sympathize with the sentiment behind them. 

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