debugserver: Propagate environment in launch-mode (pr35671)

Summary:
Make sure we propagate environment when starting debugserver with a pre-loaded
inferior. AFAIK, RNBRunLoopLaunchInferior is only called in pre-loaded inferior
scenario, so we can just pick up the debugserver environment instead of trying
to construct an envp from the (empty) context.

This makes debugserver pass an test added for an equivalent lldb-server fix.

Reviewers: jasonmolenda, clayborg

Subscribers: JDevlieghere, lldb-commits

Differential Revision: https://reviews.llvm.org/D41352

llvm-svn: 321355