[clang][Interp] Fix variadic member functions

For variadic member functions, the way we calculated the instance
pointer and RVO pointer offsts on the stack was incorrect, due
to Func->getArgSize() not returning the full size of all the
passed arguments. When calling variadic functions, we need
to pass the size of the passed (variadic) arguments to the Call*
ops, so they can use that information to properly check the
instance pointer, etc.

This patch adds a bit of code duplication in Interp.h, which I
will get rid of in later cleanup NFC patches.