libstdc++: Use C++23 deducing this in std::bind_front

libstdc++: Use C++23 deducing this in std::bind_front

This simplifies the operator() of _Bind_front using C++23 deducing
this, allowing us to condense multiple operator() overloads into one.

In passing I think we can remove _Bind_front's defaulted special member
declarations and just let the compiler implicitly generate them for us.

libstdc++-v3/ChangeLog:

	* include/std/functional (_Bind_front): Remove =default special
	member function declarations.
	(_Bind_front::operator()): Implement using C++23 deducing this
	when available.
	* testsuite/20_util/function_objects/bind_front/111327.cc:
	Adjust testcase to expect better errors in C++23 mode.