Fixed FileSpec's operator == to deal with equivalent paths such as "/tmp/a.c"

and "/private/tmp/a.c". This was done by adding a "mutable bool m_is_resolved;"
member to FileSpec and then modifying the equal operator to check if the
filenames are equal, and if they are, then check the directories. If they are
not equal, then both paths are checked to see if they have been resolved. If
they have been resolved, we resolve the paths in temporary FileSpec objects
and set each of the m_is_resolved bools to try (for lhs and rhs) if the paths
match what is contained in the path. This allows us to do more intelligent
compares without having to resolve all paths found in the debug info (which
can quickly get costly if the files are on remote NFS mounts).

llvm-svn: 118387