Two changes:

1. Use isValid() to check validity of the resulting path name in the
   eraseSuffix even though we can't think of a case where eraseSuffix could
   possibly cause an invalid path name.
2. Rewrite isValid() to not use the deprecated realpath function any more.
   It now just uses isascii to make sure all the characters are legit.

llvm-svn: 22359