[NativeProcessLinux] Remove the post-stop lambda

Summary:
The lambda was always calling SetState(eStateStopped) with small variations, so I have inlined
the code. Given that we don't have the TSC anymore, I believe we don't need to be so generic.

The only major change here is the way we choose a stop reason thread when we're interrupting a
program on client request. Previously, we were setting a null stop reason for all threads and
then fixing up the reason for one victim thread in the lambda. Now, I make sure the stop reason
is set for the victim thread correctly in the first place.

I also take the opportunity to rename CallAfter* functions into something more appropriate.

Test Plan: All tests continue to pass.

Reviewers: chaoren, vharron

Subscribers: lldb-commits

Differential Revision: http://reviews.llvm.org/D9321

llvm-svn: 236595