Fix crash if a submodule @imports another submodule from the same module. The
test also adds FIXMEs for a number of places where imports and includes of
submodules don't work very well.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@193005 91177308-0d34-0410-b5e6-96231b3b80d8