os.path.join does not always work for paths on remote platforms.

Summary:
Since we don't yet have remote windows debugging, it should be safe to assume
that the remote target uses unix path separators.

Reviewers: ovyalov, zturner, clayborg, vharron

Reviewed By: vharron

Subscribers: lldb-commits

Differential Revision: http://reviews.llvm.org/D9633

llvm-svn: 237006