Fixing a "multiple rules generate X" warning from ninja

CMake's Ninja generator was not detecting that test/lit.site.cfg.in and
test/subdir/../lit.site.cfg.in were really the same file. Ninja noticed this
and complained as both appeared as targets (for the missing file rule). Now
canonicalizing the path to ensure the paths presented to CMake are identical
and the duplication is now fixed.

llvm-svn: 175744