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

[Ilya Biryukov via cfe-commits]

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@@ -5654,6 +5654,8 @@ def warn_func_template_missing : Warning<"instantiation of function %q0 "
   InGroup<UndefinedFuncTemplate>, DefaultIgnore;
 def note_forward_template_decl : Note<
   "forward declaration of template entity is here">;
+def note_unreachable_template_decl
+    : Note<"declaration of template entity is unreachable here">;
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ilya-biryukov wrote:

I guess `"unreachable declaration of template entity is here" ` is a more natural choice.

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