Use native ::pid_t in Linux ProcessMonitor (local debugging).

There were a few places where we were not catching the possibility of negative
error codes in waitpid() calls.  This change fixes those remaining after
the llgs branch fixes to ProcessMonitor.

Change by Shawn Best.


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@212107 91177308-0d34-0410-b5e6-96231b3b80d8