Guard for null result from ClangModuleUnit::getClangModule().
Guard for null result from ClangModuleUnit::getClangModule().

The documentation for this method states that this can return null if
the module unit represents [the] imported headers.

We have a crash report on dereferencing the result of this function
farther down, and I suspect it's hitting exactly that
case. Unfortunately we only have a back trace and no test case.

Fixes: rdar://problem/41888568

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