[LLGS] Spawned process handling cleanup

Summary:
This commit moves the m_spawned_pids member from the common LLGS/Platform class to the plaform
specific part. This enables us to remove LLGS code, which was attempting to manage the
m_spawned_pids contents, but at the same time making sure, there is only one debugged process. If
we ever want to do multi-process debugging, we will probably want to replace this with a set of
NativeProcessProtocolSP anyway. The only functional change is that support for
qKillSpawnedProcess packet is removed from LLGS, but this was not used there anyway (we have the
k packet for that).

Reviewers: ovyalov, clayborg

Subscribers: lldb-commits

Differential Revision: http://reviews.llvm.org/D11557

llvm-svn: 243513