Reimplement the handling of the "current object" in designator

initializers, so that we are within the appropriate subobject after
we've processed a multi-designator designation. We're matching GCC and
EDG's behavior on all examples I've found thus far.

*Huge* thanks to Eli Friedman for pointing out my fundamental
misunderstanding of "current object" in the C99 spec.

llvm-svn: 62812