Fix PathMappingList::FindFile to handle relative incoming FileSpecs.

An equivalent change was made to RemapPaths, but it needed to be made 
here as well.  Also added a test for this and made the setup a little
more complex to avoid false successes.

<rdar://problem/47642498>

Differential Revision: https://reviews.llvm.org/D57552

llvm-svn: 353243