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
(rather than 'open')
This attribute was originally introduced for Swift classes exposed to
Objective-C, but if it /is/ used on a pure Objective-C class Swift
should respect it. There might be an actual reason, such as a hot code
path in ContrivedExampleKit checking against the class object's
address instead of using -isKindOfClass:.
For source compatibility, only enforce this in Swift 5 mode.
rdar://problem/33971529
