libstdc++: Make make-unsigned-like-t<__int128> work [PR 96042]

As well as ensuring that numeric_limits<__int128> is defined, we need to
ensure that make-unsigned-like-t and to-unsigned-like work correctly for
128-bit integers in strict mode. This ensures that a subrange created
from an iota_view's iterator and sentinel can represent its size.

Co-authored-by: Patrick Palka  <ppalka@redhat.com>

libstdc++-v3/ChangeLog:

2020-08-19  Jonathan Wakely  <jwakely@redhat.com>
	    Patrick Palka  <ppalka@redhat.com>

	PR libstdc++/96042
	* include/bits/range_access.h (__detail::__to_unsigned_like):
	Do not use make_unsigned_t<T> in the return type, as it can
	result in an error before the integral<T> constraint is checked.
	[__STRICT_ANSI__]: Add overloads for 128-bit integer types.
	(__detail::__make_unsigned_like_t): Define as the return type
	of __to_unsigned_like.
	* testsuite/std/ranges/subrange/96042.cc: New test.

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