Added support for resolving symbolic links to FileSpec.

We use the symbolic link to resolver to find the target of the LLDB shlib
symlink if there is a symlink.  This allows us to find shlib-relative  resources
even when running under the testsuite, where _lldb.so is a symlink in the Python
resource directory.

Also changed a comment to be slightly more clear about what resolve_path in the
constructor for FileSpec means, since if we were actually using realpath() this
code wouldn't have been necessary.

http://reviews.llvm.org/D12984

llvm-svn: 248048