IRgen/C++: When mark vtables used, make sure to still append to the VTableUse...

IRgen/C++: When mark vtables used, make sure to still append to the VTableUse array if we promote a vtable from being just used to having its definition required. This ensures that we properly inform the consumer about whether the vtable is required or not, previously we could fail to do so when the vtable was in the VTableUses array before the decl which marked it as required.
 - I think this can be cleaned up, since this means we may notify the consumer about the vtable twice, but I didn't see an easy fix for this without more substantial refactoring.
 - Doug, please review!

llvm-svn: 104577