Frontend: Disentangle removePathTraversal from concatenating paths

This reimplements part of r211303 in a bit of a cleaner way. Doing so
allows us to use a proper absolute path when calling addFileMapping
rather than relying on a substring being one, which should fix the
tests on Windows.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@211338 91177308-0d34-0410-b5e6-96231b3b80d8