Add a test that ensures the Clang driver behaves itself when the PATH
environment variable is changed to strange things out from under it.
Prior to r223099 in LLVM, these test cases would crash in various ways
(assert fails, stack exhaustion, etc.).

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@223100 91177308-0d34-0410-b5e6-96231b3b80d8