Canonicalize path names in the file manager before performing a lookup

Canonicalize path names in the file manager before performing a lookup
on that name. Canonicalization eliminates silliness such as "." and
"foo/.." that breaks the uniquing of files in the presence of virtual
files or files whose inode numbers have changed during
parsing/re-parsing. c-index-test isn't able to create this crazy
situation, so I've resorted to testing outside of the Clang
tree. Fixes <rdar://problem/8928220>.

Note that this hackery will go away once we have a real virtual file
system on which we can layer FileManager; the virtual-files hack is
showing cracks.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@124754 91177308-0d34-0410-b5e6-96231b3b80d8