<rdar://problem/14728845>

Making the --timeout (-t) option to "platform shell" work for real
Debug Info: move DIScope::getContext back from DwarfDebug.

This partially reverts r190330. DIScope::getContext now returns DIScopeRef
instead of DIScope. We construct a DIScopeRef from DIScope when we are
dealing with subprogram, lexical block or name space.