[C23] AST equivalence of attributes

Implicitly declared types (like __NSConstantString_tag, etc) will be
declared with visibility attributes. This causes problems when merging
ASTs because we currently reject declaration merging for declarations
with attributes.

This relaxes that restriction somewhat; implicit declarations can now
have attributes when merging; we assume that if the compiler generated
it, it's fine.

Author: AaronBallman

clang/lib/AST/ASTStructuralEquivalence.cpp

@@ -456,7 +456,9 @@ CheckStructurallyEquivalentAttributes(StructuralEquivalenceContext &Context,
                                       const Decl *D1, const Decl *D2,
                                       const Decl *PrimaryDecl = nullptr) {
   // If either declaration has an attribute on it, we treat the declarations
-  // as not being structurally equivalent.
+  // as not being structurally equivalent unless both declarations are implicit
+  // (ones generated by the compiler like __NSConstantString_tag).
+  //
   // FIXME: this should be handled on a case-by-case basis via tablegen in
   // Attr.td. There are multiple cases to consider: one declaration with the
   // attribute, another without it; different attribute syntax|spellings for
@@ -468,7 +470,7 @@ CheckStructurallyEquivalentAttributes(StructuralEquivalenceContext &Context,
     D1Attr = *D1->getAttrs().begin();
   if (D2->hasAttrs())
     D2Attr = *D2->getAttrs().begin();
-  if (D1Attr || D2Attr) {
+  if ((D1Attr || D2Attr) && !D1->isImplicit() && !D2->isImplicit()) {
     const auto *DiagnoseDecl = cast<TypeDecl>(PrimaryDecl ? PrimaryDecl : D2);
     Context.Diag2(DiagnoseDecl->getLocation(),
                   diag::warn_odr_tag_type_with_attributes)