[clang] [Clang][Sema] Improve support for explicit specializations of constrained member functions & member function templates (PR #88963)

[Erich Keane via cfe-commits]

================
@@ -5437,6 +5437,11 @@ def note_function_template_spec_matched : Note<
 def err_function_template_partial_spec : Error<
     "function template partial specialization is not allowed">;
 
+def err_function_member_spec_ambiguous : Error<
+    "ambiguous member function specialization of %q0">;
----------------
erichkeane wrote:

```suggestion
    "ambiguous member function specialization of %0">;
```

https://github.com/llvm/llvm-project/pull/88963