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
Method types are now required to be `(Self) -> (Args...) -> Return`, not
`Self -> (Args...) -> Return`.
See also https://github.com/apple/swift/pull/9454 .
Fixes rdar://problem/32588152 .
