[modules] Fix tracking ObjCInterfaceType decl when there are multiple definitions.

With the old approach we were updating `ObjCInterfaceType.Decl` to the
last encountered definition. But during loading modules
`ASTDeclReader::VisitObjCInterfaceDecl` keeps the *first* encountered
definition. So with multiple definitions imported there would be a
disagreement between expected definition in `ObjCInterfaceType.Decl` and
actual definition `ObjCInterfaceDecl::getDefinition` which can lead to
incorrect diagnostic.

Fix by not tracking definition in `ObjCInterfaceType` explicitly but by
getting it from redeclaration chain.

Partially reverted 919fc500 keeping the
modified test case as the correct behavior is achieved in a different
way.

Differential Revision: https://reviews.llvm.org/D110452