fix rdar://8340348, a miscompile of boost that was exposed by r109848.

That revision started classifying truly empty structs like "Y" and "X"
as being NoClass/NoClass and turning them into 'ignore'.  The call code
turns around and allocates space for the ignored argument with 
GetUndefRValue.  The bug is that GetUndefRValue would return the address
as undef, instead of returning an object with a defined address but 
undefined contents.

llvm-svn: 111794