c++: allow variadic operator[] for C++23 [PR103460]

c++: allow variadic operator[] for C++23 [PR103460]

wg21.link/p2128 removed "with exactly one parameter" from over.sub
section.  grok_op_properties has for that the last 2 lines in:
    case OVL_OP_FLAG_BINARY:
      if (arity != 2)
        {
          if (operator_code == ARRAY_REF && cxx_dialect >= cxx23)
            break;
but unfortunately it isn't enough, we reject variadic operator[]
earlier.  The following patch accepts variadic operator[] for C++23
too.

2022-03-10  Jakub Jelinek  <jakub@redhat.com>

	PR c++/103460
	* decl.cc (grok_op_properties): Allow variadic operator[] for
	C++23.

	* g++.dg/cpp23/subscript7.C: New test.