[windows] LLDB shows the wrong values when register read is executed at a frame other than zero

Summary:
This is a clean version of the change suggested here: https://bugs.llvm.org/show_bug.cgi?id=37495

The main change is to follow the same pattern as non-windows targets and use an unwinder object to retrieve the register context. I also changed a couple of the comments to actually log, so that issues with unsupported scenarios can be tracked down more easily. Lastly, ClearStackFrames is implemented in the base class, so individual thread implementations don't have to override it.

Reviewers: asmith, zturner, aleksandr.urakov

Reviewed By: aleksandr.urakov

Subscribers: emaste, stella.stamenova, tatyana-krasnukha, llvm-commits

Differential Revision: https://reviews.llvm.org/D49111

llvm-svn: 336732