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
Previously we set up the diagnostic options too early for these flags
to be honored, in order to diagnose problems in parsing the arguments
being passed to Clang. The solution is to decide that that diagnostic
engine is a temporary one and to create a proper one later. (And if
this sounds like a lousy solution, I don't disagree, but it's what
Clang does too.)
rdar://problem/39197895 and possibly something older but I can't find it
