Convert last use of PathV1.h in Compilation.cpp

The way we decide which file to remove is fairly odd. I took a quick look at
maybe changing that, but it would be a more work than I want to put at this
right now, so I left pair of FIXMEs.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@184766 91177308-0d34-0410-b5e6-96231b3b80d8