This enables additional tests for the ClangImporter. This found a
missing piece in the `-enable-objc-interop` work that was done
previously. Address that and enable the tests. There are now the
following failing tests on Linux:
* sdk - depends on Foundation (not hermetic, see SR-7572)
* mixed-nsuinteger - depends on Foundation (not hermetic, see SR-7572)
* import-mixed-with-header-twice - requires apple/swift PR#16022
* can_import_objc_idempotent - requires apple/swift PR#16022
* objc_protocol_renaming - requires apple/swift PR#16022
For the Optional<Module *> returned by getClangSubmoduleForDecl, the
outside Optional specifies whether there's an answer at all. That
answer can still be null if the declaration comes from a bridging
header. In this particular case, we're guaranteed to get an answer,
but that answer may be null.
rdar://problem/32463543
