With the compiler bug fixed we can simplify the __sizable concept to use
a return-type-requirement again. I also realised it was redundantly
re-checking a subset of the sized_sentinel_for requirements.

The compiler fix also revealed bugs in two tests which started to fail
and are fixed by this patch.

	* include/bits/range_access.h (__sizable): Rename to __sentinel_size.
	Remove workaround for PR c++/92268 and remove redundant requirements
	that are already checked by sized_sentinel_for.
	* testsuite/std/ranges/access/cend.cc: Fix failures.
	* testsuite/std/ranges/access/end.cc: Likewise.

From-SVN: r277667