libstdc++: Add always_inline to ranges iterator ops and access functions

libstdc++: Add always_inline to ranges iterator ops and access functions

Most of the basis operations for ranges such as ranges::begin and
ranges::next are trivial one-line function bodies, so can be made
always_inline to reduce the abstraction penalty for -O0 code.

Now that we no longer need to support the -fconcepts-ts grammar, we can
also move some [[nodiscard]] attributes to the more natural position
before the function declaration, instead of between the declarator-id
and the function parameters, e.g. we can use:

  template<typename T> requires C<T> [[nodiscard]] auto operator()(T&&)

instead of:

  template<typename T> requires C<T> auto operator() [[nodiscard]] (T&&)

The latter form was necessary because -fconcepts-ts used a different
grammar for the requires-clause, parsing 'C<T>[[x]]' as a subscripting
operator with an ill-formed argument '[x]'. In the C++20 grammar you
would need to use parentheses to use a subscript in a constraint, so
without parentheses it's parsed as an attribute.

libstdc++-v3/ChangeLog:

	* include/bits/ranges_base.h (__detail::__to_unsigned_like)
	(__access::__possible_const_range, __access::__as_const)
	(__distance_fn::operator(), __next_fn::operator())
	(__prev_fn::operator()): Add always_inline attribute.
	(_Begin::operator(), _End::operator(), _RBegin::operator())
	(_REnd::operator(), _Size::operator(), _SSize::operator())
	(_Empty::operator(), _Data::operator(), _SSize::operator()):
	Likewise. Move nodiscard attribute to start of declaration.

Reviewed-by: Tomasz Kamiński <tkaminsk@redhat.com>