The implementation of P2091R0 was incomplete, so that some range access
CPOs used perfect forwarding where they should not. This fixes it by
consistently operating on lvalues.

Some additional changes that are not necessary to fix the bug:

Modify the __as_const helper to simplify its usage. Instead of deducing
the value category from its argument, and requiring callers to forward
the argument as the correct category, add a non-deduced template
parameter which is used for the value category and accept the argument
as an lvalue. This means callers say __as_const<T>(t) instead of
__as_const(std::forward<T>(t)).

Always use an lvalue reference type as the template argument for the
_S_noexcept helpers, so that we only instantiate one specialization for
lvalues and rvalues of the same type.

Move some helper concepts and functions from namespace std::__detail
to ranges::__cust_access, to be consistent with the ranges::begin CPO.
This ensures that the __adl_begin concept and the _Begin::operator()
function are in the same namespace, so unqualified lookup is consistent
and the poison pills for begin are visible to both.

Simplified static assertions for arrays, because the expression a+0 is
already ill-formed for an array of incomplete type.

Signed-off-by: Jonathan Wakely <jwakely@redhat.com>

libstdc++-v3/ChangeLog:

	PR libstdc++/100824
	* include/bits/iterator_concepts.h (__detail::__decay_copy)
	(__detail::__member_begin, __detail::__adl_begin): Move to
	namespace ranges::__cust_access.
	(__detail::__ranges_begin): Likewise, and rename to __begin.
	Remove redundant static assertion.
	* include/bits/ranges_base.h (_Begin, _End, _RBegin, _REnd):
	Use lvalue in noexcept specifier.
	(__as_const): Add non-deduced parameter for value category.
	(_CBegin, _CEnd, _CRBegin, _CREnd, _CData): Adjust uses of
	__as_const.
	(__member_size, __adl_size, __member_empty, __size0_empty):
	(__eq_iter_empty, __adl_data): Use lvalue objects in
	requirements.
	(__sentinel_size): Likewise. Add check for conversion to
	unsigned-like.
	(__member_data): Allow non-lvalue types to satisfy the concept,
	but use lvalue object in requirements.
	(_Size, _SSize): Remove forwarding to always use an lvalue.
	(_Data): Likewise. Add static assertion for arrays.
	* testsuite/std/ranges/access/cdata.cc: Adjust expected
	behaviour for rvalues. Add negative tests for ill-formed
	expressions.
	* testsuite/std/ranges/access/data.cc: Likewise.
	* testsuite/std/ranges/access/empty.cc: Adjust expected
	behaviour for rvalues.
	* testsuite/std/ranges/access/size.cc: Likewise.

Tested powerpc64le-linux. Committed to trunk.

I think this should be backported to gcc-11 too, and maybe gcc-10 too.