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
We just import this as a property, and Swift doesn't support
properties with dynamic Self type, even if they are read-only. Don't
mark the getter as returning dynamic Self in this case.
(This was only a problem in builds with the AST verifier turned on.)
https://bugs.swift.org/browse/SR-5684
