[LLGS] Avoid misrepresenting log lines as inferior output

Summary:
in case we are logging to stdout, any log lines from the forked child can be misconstrued to be
inferior output. To avoid this, we disable all logging immediately after forking.

I also fix the implementatoion of DisableAllLogChannels, which was a no-op before this commit.

Reviewers: clayborg, ovyalov

Subscribers: dean, lldb-commits

Differential Revision: http://reviews.llvm.org/D12083

llvm-svn: 245272