Fix race condition with -o "process launch" on linux

Summary:
starting a debug session on linux with -o "process launch" lldb parameter was failing since
Target::Launch (in sychronous mode) is expecting to be able to receive public process events.
However, PlatformLinux did not set up event hijacking on process launch, which caused these
events to be processed elsewhere and left Target::Launch hanging. This patch enables event
interception in PlatformLinux (which was commented out).

Upon enabling event interception, I noticed an issue, which I traced back to the inconsistent
state of public run lock, which remained false even though public and private process states were
"stopped". I addressed this by making sure the run lock is "stopped" upon exit from
WaitForProcessToStop (which already had similar provisions for other return paths).

Test Plan: This should fix the intermittent TestFormats failure we have been experiencing on Linux.

Reviewers: jingham, clayborg, vharron

Subscribers: lldb-commits

Differential Revision: http://reviews.llvm.org/D8079

llvm-svn: 231460