[lit] Use realpath when adding to the config map.

Since the path a user specifies to the llvm-lit script might be
different than the source tree they built from (since they could
be behind different symlinks), we need to use realpath to make
sure that path comparisons work as expected.

Even better would be to use a custom dictionary comparison with
actual file equivalence comparison semantics, but this is the
least friction to unbreak things for now.

llvm-svn: 313594