Make it possible for lldb to launch a remote binary with no local file.

We don't actually need a local copy of the main executable to debug
a remote process.  So instead of treating "no local module" as an error,
see if the LaunchInfo has an executable it wants lldb to use, and if so
use it.  Then report whatever error the remote server returns.

Differential Revision: https://reviews.llvm.org/D113521