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're trying to get rid of implicit bridging-header imports, as a feature.
These are IMPORTED_HEADER blocks left in modules built with bridging
headers, that trigger re-importing the bridging header into any client
that imports the module.
As a half-way measure to deprecating them, we add a warning here that
triggers when an implicit bridging-header import occurs that is _not_
suppressed as redundant by clang.
