Use the new llvm::sys::EnvPathSeparator.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@184851 91177308-0d34-0410-b5e6-96231b3b80d8
Use the simpler version of llvm::sys::fs::exists.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@184855 91177308-0d34-0410-b5e6-96231b3b80d8