[lldb] Identify Swift-implemented ObjC classes

Classes implemented in Swift can be exposed to ObjC. For those classes, the ObjC
metadata is incomplete. Specifically, the encoded types of the ivars are incomplete. As
one might expect, the Swift metadata _is_ complete. In such cases, the Swift runtime
should be consulted when determining the dynamic type of a value.

Differential Revision: https://reviews.llvm.org/D152837