[clang] Don't make ObjCIvarDecl visible twice when adding them to an implicit ObjCInterfaceDecl

[teemperor]

Summary

addDecl is making the ivar visible in its primary context. The primary context of the ivar here is
in a 'fragile' ABI the ObjCInterfaceDecl and in a 'non-fragile' ABI the current ObjCImplementationDecl.
The additional call to makeDeclVisibleInContext to make the ivar visible in the ObjCInterfaceDecl
is only necessary in the 'non-fragile' case (as in the 'fragile' case the Decl becomes automatically visible
in the ObjCInterfaceDecl with the addDecl call as thats its primary context). See Sema::ActOnIvar
for where the ivar is put into a different context depending on the ABI.

To put this into an example:

@implementation SomeClass
{
  id ivar1;
}
@end

fragile case:
implicit ObjCInterfaceDecl 'SomeClass'
`- ivar1 (in primary context and will be automatically made visible)
ObjCImplementationDecl 'SomeClass'

non-fragile case:
implicit ObjCInterfaceDecl 'SomeClass'
`-<<<ivar1 not visible here and needs to be manually marked as visible.>>>
ObjCImplementationDecl 'SomeClass'
`- ivar1 (in its primary context and will be automatically made visible here)

Making a Decl visible multiple times in the same context is inefficient and potentially
can lead to crashes. See D84827 for more info and what this is breaking.

[teemperor]

That code is <2010 code and not sure who's the author is, so just adding Adrian as the reviewer (who already got a similar review)

[teemperor]

• Preserve the makeDeclVisibleInContext call for non-fragile ABIs.

clang/lib/Sema/SemaDeclObjC.cpp

void Sema::CheckImplementationIvars(ObjCImplementationDecl *ImpDecl,
   /// Check case of non-existing \@interface decl.
   /// (legacy objective-c \@implementation decl without an \@interface decl).
   /// Add implementations's ivar to the synthesize class's ivar list.
   if (IDecl->isImplicitInterfaceDecl()) {
     IDecl->setEndOfDefinitionLoc(RBrace);
     // Add ivar's to class's DeclContext.
     for (unsigned i = 0, e = numIvars; i != e; ++i) {
       ivars[i]->setLexicalDeclContext(ImpDecl);
+      // In a 'fragile' runtime the ivar was added to the implicit
+      // ObjCInterfaceDecl while in a 'non-fragile' runtime the ivar is
+      // only in the ObjCImplementationDecl. In the non-fragile case the ivar
+      // therefore also needs to be propagated to the ObjCInterfaceDecl.
+      if (!LangOpts.ObjCRuntime.isFragile())
         IDecl->makeDeclVisibleInContext(ivars[i]);
       ImpDecl->addDecl(ivars[i]);
     }
 
     return;
   }
   // If implementation has empty ivar list, just return.
   if (numIvars == 0)
     return;