[modules] Add testcase for a bug reduced from a selfhost issue. This bug was
never present in Clang trunk, but was present in some of my development work,
and it seems like a useful test to have.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@214154 91177308-0d34-0410-b5e6-96231b3b80d8