c++: Make sure fold_sizeof_expr returns the correct type [PR117775]

c++: Make sure fold_sizeof_expr returns the correct type [PR117775]

We currently ICE upon the following code, that is valid under
-Wno-pointer-arith:

=== cut here ===
int main() {
  decltype( [](auto) { return sizeof(void); } ) x;
  return x.operator()(0);
}
=== cut here ===

The problem is that "fold_sizeof_expr (sizeof(void))" returns
size_one_node, that has a different TREE_TYPE from that of the sizeof
expression, which later triggers an assert in cxx_eval_store_expression.

This patch makes sure that fold_sizeof_expr always returns a tree with
the size_type_node type.

	PR c++/117775

gcc/cp/ChangeLog:

	* decl.cc (fold_sizeof_expr): Make sure the folded result has
	type size_type_node.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/constexpr-117775.C: New test.