[clang][Interp] Fix discarding void-typed comma expressions

First, we need to handle void types in visitExpr, so we don't run into
an assertion there when we try to pop a return value from the stack
that isn't there.

Secondly, we need to handle it when visiting comma expressions so we
don't do the same thing there.

Differential Revision: https://reviews.llvm.org/D148925