[clang][Interp] Remove args from called functions in more cases

When calling functions in the checkingPotentialConstantExpression mode,
we cannot have arguments (including This + RVO pointers) for the
toplevel callee, but the functions called from within can work just
fine, or at least we succeed in pushing their arguments on the stack, so
we must also succeed in removing them again.

Differential Revision: https://reviews.llvm.org/D150358