[LLGS] Avoid bogus error message on process termination

Summary:
Handle_k was printing an error when killing a process because KillSpawnedProcess was expecting to
be asynchronously notified of the process death, which no longer works, since we don't wait for
the process on a separate thread. However, the whole usage of KillSpawnedProcess is dubious here,
since it tries to be nice and terminate the process first with SIGTERM, which will not have the
intended effect on a ptraced process. I replace this code with a call to
NativeProcessProtocol::Kill, which does not suffer from these problems.

Reviewers: chaoren, ovyalov

Subscribers: lldb-commits

Differential Revision: http://reviews.llvm.org/D11520

llvm-svn: 243397