Certain directives are unsupported on Windows (some of which could/should be
supported). We would not diagnose the use but rather crash during the emission
as we try to access the Target Streamer. Add an assertion to prevent creating a
NULL reference (which is not permitted under C++) as well as a test to ensure
that we can diagnose the disabled directives.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@218014 91177308-0d34-0410-b5e6-96231b3b80d8
