Use temporary module cache in test

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@212467 91177308-0d34-0410-b5e6-96231b3b80d8
Driver: Fix think-o in adding -ivfsoverlay flag to crashdumps

If there isn't a VFS to overlay we shouldn't be adding a -ivfsoverlay
flag.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@212468 91177308-0d34-0410-b5e6-96231b3b80d8