We use the just-built clang without any -target or -sdk flags. Keep things
simple and limit the test to macOS for now.
Fixes <rdar://problem/50586614>.
The objc_loadClassref() symbol is weak linked from Clang now, so we can
un-XFAIL this test. Note that we have to pass a special flag to Clang
since objc_loadClassref() is not in the SDK's TBD files yet.
Also, change the test to use its own asserts instead of FileCheck since
there's no output unless a new libobjc is available.
