[clang] [clang] Reword apologetic Clang diagnostic messages (PR #76310)

[via cfe-commits]

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@@ -5188,7 +5188,7 @@ def err_template_arg_not_object_or_func : Error<
 def err_template_arg_not_pointer_to_member_form : Error<
   "non-type template argument is not a pointer to member constant">;
 def err_template_arg_member_ptr_base_derived_not_supported : Error<
-  "sorry, non-type template argument of pointer-to-member type %1 that refers "
+  "non-type template argument of a pointer to member type %1, that refers "
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cor3ntin wrote:

`pointer-to-member` is a term of art, we should keep it (and `a` is incorrect grammatically)

https://github.com/llvm/llvm-project/pull/76310