Clang: Add warning flag for storage class specifiers on explicit specializations

With the recent fix for this situation in class members (#93873) (for
which the fixed code is invalid prior to this patch - making migrating
code difficult as it must be in lock-step with the compiler migration,
if building with -Werror) it'd be really useful to be able to disable
this warning during the compiler migration/decouple the compiler
migration from the source fixes.

In theory this approach will regress the codebase to the previous
non-member cases of this issue that were already being held back by the
warning (as opposed to if we carved out the new cases into a separate
warning from the existing cases) but I think this'll be so rare and the
cleanup so simple, that the extra regressions of disabling the warning
broadly won't be too much of a problem. (but if folks disagree, I'm open
to making the warning more fine-grained)

Author: dwblaikie

clang/include/clang/Basic/DiagnosticGroups.td

@@ -1535,3 +1535,7 @@ def BitIntExtension : DiagGroup<"bit-int-extension">;
 
 // Warnings about misuse of ExtractAPI options.
 def ExtractAPIMisuse : DiagGroup<"extractapi-misuse">;
+
+// Warnings about using the non-standard extension having an explicit specialization
+// with a storage class specifier.
+def ExplicitSpecializationStorageClass : DiagGroup<"explicit-specialization-storage-class">;

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -5366,7 +5366,7 @@ def err_not_class_template_specialization : Error<
   "cannot specialize a %select{dependent template|template template "
   "parameter}0">;
 def ext_explicit_specialization_storage_class : ExtWarn<
-  "explicit specialization cannot have a storage class">;
+  "explicit specialization cannot have a storage class">, InGroup<ExplicitSpecializationStorageClass>;
 def err_dependent_function_template_spec_no_match : Error<
   "no candidate function template was found for dependent"
   " %select{member|friend}0 function template specialization">;

clang/test/Misc/warning-flags.c

@@ -18,10 +18,9 @@ This test serves two purposes:
 
 The list of warnings below should NEVER grow.  It should gradually shrink to 0.
 
-CHECK: Warnings without flags (66):
+CHECK: Warnings without flags (65):
 
 CHECK-NEXT:   ext_expected_semi_decl_list
-CHECK-NEXT:   ext_explicit_specialization_storage_class
 CHECK-NEXT:   ext_missing_whitespace_after_macro_name
 CHECK-NEXT:   ext_new_paren_array_nonconst
 CHECK-NEXT:   ext_plain_complex