The Clang Importer when C++ interop is not enabled, disambigate an Obj-C
class and protocol that are named the same by appending `Protocol` to
the protocol. This was not happening when C++ interop was enabled, but
should also apply to Obj-C++ modules.
The fix is providing an starting scope for the search, which the C++
name lookup need to actually find the similarly named counterpart.
Includes a test to avoid this problem creeping in again, and locally it
did not break any other tests.
