[Windows] Add support of watchpoints to `ProcessWindows`

Summary:
This patch adds support of watchpoints to the old `ProcessWindows` plugin.

The `ProcessWindows` plugin uses the `RegisterContext` to set and reset
watchpoints. The `RegisterContext` has some interface to access watchpoints,
but it is very limited (e.g. it is impossible to retrieve the last triggered
watchpoint with it), that's why I have implemented a slightly different
interface in the `RegisterContextWindows`. Moreover, I have made the
`ProcessWindows` plugin responsible for search of a vacant watchpoint slot,
because watchpoints exist per-process (not per-thread), then we can place
the same watchpoint in the same slot in different threads. With this scheme
threads don't need to have their own watchpoint lists, and it simplifies
identifying of the last triggered watchpoint.

Reviewers: asmith, stella.stamenova, amccarth

Reviewed By: amccarth

Subscribers: labath, zturner, leonid.mashinskiy, abidh, JDevlieghere, lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D67168

llvm-svn: 371166