libstdc++: Make std::function deduction guide support explicit object functions [PR113335]

This makes the deduction guides for std::function and std::packaged_task
work for explicit object member functions, i.e. "deducing this", as per
LWG 3617.

libstdc++-v3/ChangeLog:

	PR libstdc++/113335
	* include/bits/std_function.h (__function_guide_helper): Add
	partial specialization for explicit object member functions, as
	per LWG 3617.
	* testsuite/20_util/function/cons/deduction_c++23.cc: Check
	explicit object member functions.
	* testsuite/30_threads/packaged_task/cons/deduction_c++23.cc:
	Likewise.