Don't use PathV1.h in Tools.cpp.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@184856 91177308-0d34-0410-b5e6-96231b3b80d8
Use simpler version of llvm::sys::fs::exists.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@184857 91177308-0d34-0410-b5e6-96231b3b80d8