libstdc++: integer-class types as per [iterator.concept.winc]

This implements signed and unsigned integer-class types, whose width is
one bit larger than the widest supported signed and unsigned integral
type respectively.  In our case this is either __int128 and unsigned
__int128, or long long and unsigned long long.

Internally, the two integer-class types are represented as a largest
supported unsigned integral type plus one extra bit.  The signed
integer-class type is represented in two's complement form with the
extra bit acting as the sign bit.

libstdc++-v3/ChangeLog:

	* include/Makefile.am (bits_headers): Add new header
	<bits/max_size_type.h>.
	* include/Makefile.in: Regenerate.
	* include/bits/iterator_concepts.h
	(ranges::__detail::__max_diff_type): Remove definition, replace
	with forward declaration of class __max_diff_type.
	(__detail::__max_size_type): Remove definition, replace with
	forward declaration of class __max_size_type.
	(__detail::__is_unsigned_int128, __is_signed_int128)
	(__is_int128): New concepts.
	(__detail::__is_integer_like): Accept __int128 and unsigned
	__int128.
	(__detail::__is_signed_integer_like): Accept __int128.
	* include/bits/max_size_type.h: New header.
	* include/bits/range_access.h: Include <bits/max_size_type.h>.
	(__detail::__to_unsigned_like): Two new overloads.
	* testsuite/std/ranges/iota/difference_type.cc: New test.
	* testsuite/std/ranges/iota/max_size_type.cc: New test.

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