[lldb] [Platform] Move common ::DebugProcess() to PlatformPOSIX

Move common ::DebugProcess() implementation shared by Linux and NetBSD
(and to be shared by FreeBSD shortly) into PlatformPOSIX, and move
the old base implementation used only by Darwin to PlatformDarwin.

Differential Revision: https://reviews.llvm.org/D88852