Limit scope of RegisterContextPOSIX.h header

RegisterContextPOSIX.h is poorly named and contains only the declaration
of POSIXBreakpointProtocol, which is used for in-process live kernel
debugging. It is now relevant only to FreeBSD.

In source/Plugins/Process/Utility/RegisterContext*.h (after assorted
rework and refactoring) it only served the purpose of #including other
necessary headers as a side-effect. Remove it from them and just include
the required headers directly.

Differential Revision: http://reviews.llvm.org/D12830

llvm-svn: 247558