Simplify core file register context handling

This seems like a more straightforward approach for instantiating the
appropriate RegisterContext* class, made possible by Michael Sartain's
cleanup of the POSIX RegisterContext class hierarchy in r190647.

Better (user-facing) error handling is still needed.


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@201457 91177308-0d34-0410-b5e6-96231b3b80d8