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

`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:

```
lang=c++
@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.

Reviewed By: aprantl

Differential Revision: https://reviews.llvm.org/D84829