Change this to an IR test instead of an assembly test (as per comment in r171134).

This makes the test not dependent on LLVM & won't vary/break based on LLVM
codegen related changes. Appropriately testing at the Clang level what was
fixed at the Clang level originally (in r124210).

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@171175 91177308-0d34-0410-b5e6-96231b3b80d8