Use non-blocking waitpid in NativeProcessLinux

Summary:
This is the first phase of the merging of Monitor and Operation threads in NativeProcessLinux
(which is necessary since the two threads race inside Linux kernel). Here, I reimplement the
Monitor thread do use non-blocking waitpid calls, which enables later addition of code from the
operation thread.

Test Plan: Ran the test suite a couple of times, no regressions detected.

Reviewers: vharron, ovyalov, tberghammer

Subscribers: lldb-commits

Differential Revision: http://reviews.llvm.org/D9048

llvm-svn: 235193