libstdc++: LWG 3286 ranges::size is not required to be valid after ...

libstdc++: LWG 3286 ranges::size is not required to be valid after ...

... a call to ranges::begin on an input range.

This implements LWG 3286.  The new wording for the single-argument constructor
for subrange is implemented by splitting the constructor into two delegating
constructors, one constrained by _S_store_size and the other by !_S_store_size.

Tested on x86_64-pc-linux-gnu, both added tests fail before the patch and pass
with the patch.

libstdc++-v3/ChangeLog:

	LWG 3286 ranges::size is not required to be valid after a call to
	ranges::begin on an input range
	* include/std/ranges (subrange::subrange): Split single-argument
	constructor into two, one constrained by _S_store_size and another by
	!_S_store_size.
	(take_view::begin): Call size() before calling ranges::begin(_M_base).
	* testsuite/std/ranges/adaptors/lwg3286.cc: New test.
	* testsuite/std/ranges/subrange/lwg3286.cc: New test.