[NativeProcessLinux] Integrate MainLoop

Summary:
This commit integrates MainLoop into NativeProcessLinux. By registering a SIGCHLD handler with
the llgs main loop, we can get rid of the special monitor thread in NPL, which saves as a lot of
thread ping-pong when responding to client requests (e.g. qThreadInfo processing time has been
reduced by about 40%). It also makes the code simpler, IMHO.

Reviewers: ovyalov, clayborg, tberghammer, chaoren

Subscribers: lldb-commits

Differential Revision: http://reviews.llvm.org/D11150

This is a resubmission of r242305 after it was reverted due to bad interactions with the stdio
thread.

llvm-svn: 242783