<rdar://problem/13590152>

Providing a dummy RegisterContext to secure against faulty Python OS plugins that do not return a valid RegisterContext
The RegisterContextDummy exports a PC with a constant 0xFFFFFFFFFFFFFFFF value



git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@180033 91177308-0d34-0410-b5e6-96231b3b80d8