[CodeGen] Compute the objc EH vtable address point using inbounds GEP.

The object is provided by the objc runtime and is never visible in the
module itself, but even so, the address point we compute points into it,
and "+16" is guaranteed not to overflow.

This matches the c++ vtable IRGen.

Note that I'm not entirely convinced the 'i8*' type is correct here: at
the IR level, we're accessing memory that's outside the global object.
But we don't control the allocation, so it's not obviously wrong either.
But either way, this is only in a global initializer, so I don't think
it's going to be mucked with.  Filed PR35352 to discuss that.

llvm-svn: 318545