libstdc++: Fix exposure of TU-local lambda in __detail::__func_handle_t.

libstdc++: Fix exposure of TU-local lambda in __detail::__func_handle_t.

The lambda is considered to be TU-local entity, use a named function
instead.

As drive-by, a functor stored inside __func_handle::_Inplace is renamed
to _M_fn, as we no longer limit the functor type to function pointers.

libstdc++-v3/ChangeLog:

	* include/std/ranges (__func_handle::__select): Named function
	extracted from local lambda.
	(__detail::__func_handle_t): Define using __func_handle::__select.
	(__func_handle::_Inplace): Raname _M_ptr to _M_fn.

Reviewed-by: Jonathan Wakely <jwakely@redhat.com>
Signed-off-by: Tomasz Kamiński <tkaminsk@redhat.com>