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
Not every declaration that's syntactically in an Objective-C container
is a member of that container. Double-check the decl context before
adding it.
This is technically a breaking change in non-asserts builds, because
the struct really could be found via member lookup. But that should
be considered a bug, and I /suspect/ no one is relying on it.
rdar://problem/32451417
