[Clang] counted_by attr can apply only to C99 flexible array members (#72347)

Ensure that we're dealing only with C99 flexible array members. I.e.
ones with incomplete types:

  struct s {
    int count;
    char array[]; /* note: no size specified */
  };

Authored-by: Bill Wendling <isanbard@gmail.com>

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -6412,7 +6412,7 @@ def warn_superclass_variable_sized_type_not_at_end : Warning<
   " in superclass %3">, InGroup<ObjCFlexibleArray>;
 
 def err_counted_by_attr_not_on_flexible_array_member : Error<
-  "'counted_by' only applies to flexible array members">;
+  "'counted_by' only applies to C99 flexible array members">;
 def err_counted_by_attr_refers_to_flexible_array : Error<
   "'counted_by' cannot refer to the flexible array %0">;
 def err_counted_by_must_be_in_structure : Error<

clang/lib/AST/DeclBase.cpp

@@ -421,9 +421,6 @@ bool Decl::isFlexibleArrayMemberLike(
     using FAMKind = LangOptions::StrictFlexArraysLevelKind;
 
     llvm::APInt Size = CAT->getSize();
-    FAMKind StrictFlexArraysLevel =
-        Ctx.getLangOpts().getStrictFlexArraysLevel();
-
     if (StrictFlexArraysLevel == FAMKind::IncompleteOnly)
       return false;
 

clang/lib/Sema/SemaDeclAttr.cpp

@@ -8430,7 +8430,7 @@ bool Sema::CheckCountedByAttr(Scope *S, const FieldDecl *FD) {
   }
 
   LangOptions::StrictFlexArraysLevelKind StrictFlexArraysLevel =
-      Context.getLangOpts().getStrictFlexArraysLevel();
+      LangOptions::StrictFlexArraysLevelKind::IncompleteOnly;
 
   if (!Decl::isFlexibleArrayMemberLike(Context, FD, FD->getType(),
                                        StrictFlexArraysLevel, true)) {

clang/test/Sema/attr-counted-by.c

@@ -1,4 +1,4 @@
-// RUN: %clang_cc1 -fstrict-flex-arrays=3 -fsyntax-only -verify %s
+// RUN: %clang_cc1 -fsyntax-only -verify %s
 
 #define __counted_by(f)  __attribute__((counted_by(f)))
 
@@ -38,7 +38,12 @@ struct array_of_ints_count {
 
 struct not_a_fam {
   int count;
-  struct bar *non_fam __counted_by(count); // expected-error {{'counted_by' only applies to flexible array members}}
+  struct bar *non_fam __counted_by(count); // expected-error {{'counted_by' only applies to C99 flexible array members}}
+};
+
+struct not_a_c99_fam {
+  int count;
+  struct bar *non_c99_fam[0] __counted_by(count); // expected-error {{'counted_by' only applies to C99 flexible array members}}
 };
 
 struct annotated_with_anon_struct {