This fixes a crash in Clang.
Starting with DWARF 5 we are emitting ObjC method declarations as
children of their containing entity. This worked for interfaces, but
didn't consider the case of synthessized properties. When a property
of a protocol is synthesized in an interface implementation the
ObjCMethodDecl that was passed to CGF::StartFunction was the property
*declaration* which obviously couldn't have a containing
interface. This patch passes the containing interface all the way
through to CGDebugInfo, so the function declaration can be created
with the correct parent (= the class implementing the protocol).