c++: Implement C++23 P2589R1 - - static operator[]

Here is a patch that implements the static operator[] paper.
One thing that doesn't work properly is the same problem as I've filed
yesterday for static operator() - PR107624 - that side-effects of
the postfix-expression on which the call or subscript operator are
applied are thrown away, I assume we have to add them into COMPOUND_EXPR
somewhere after we find out that the we've chosen a static member function
operator.

2022-11-15  Jakub Jelinek  <jakub@redhat.com>

gcc/c-family/
	* c-cppbuiltin.cc (c_cpp_builtins): Bump C++23
	__cpp_multidimensional_subscript macro value to 202211L.
gcc/cp/
	* decl.cc (grok_op_properties): Implement C++23 P2589R1
	- static operator[].  Handle operator[] similarly to operator()
	- allow static member functions, but pedwarn on it for C++20 and
	older.  Unlike operator(), perform rest of checks on it though for
	C++20.
	* call.cc (add_operator_candidates): For operator[] with class
	typed first parameter, pass that parameter as first_arg and
	an adjusted arglist without that parameter.
gcc/testsuite/
	* g++.dg/cpp23/subscript9.C: New test.
	* g++.dg/cpp23/feat-cxx2b.C: Expect a newer
	__cpp_multidimensional_subscript value.
	* g++.old-deja/g++.bugs/900210_10.C: Don't expect an error
	for C++23 or later.

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