llgs: Propagate the environment when launching the inferior from command line

Summary:
We were failing to propagate the environment when lldb-server was
started with a pre-loaded process
(e.g.: lldb-server gdbserver -- inferior --inferior_args)

This patch makes sure the environment is propagated. Instead of adding a
new GDBRemoteCommunicationServerLLGS::SetLaunchEnvironment function to
complement SetLaunchArgs and SetLaunchFlags, I replace these with a
more generic SetLaunchInfo, which can be used to set any launch-related
property.

The accompanying test also verifies that the server correctly terminates
the connection after sending the exit packet (specifically, that it does
not send the exit packet twice).

Reviewers: clayborg, eugene

Subscribers: lldb-commits, mgorny

Differential Revision: https://reviews.llvm.org/D41070

llvm-svn: 320984