Fix a use-after-free bug I recently introduced in lookupModuleFile

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@207932 91177308-0d34-0410-b5e6-96231b3b80d8
Speculative fix to unbreak the buildbots that fail with compiler errors.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@207933 91177308-0d34-0410-b5e6-96231b3b80d8