libstdc++: Add noexcept to std::begin etc as per LWG 2280 and 3537

libstdc++: Add noexcept to std::begin etc as per LWG 2280 and 3537

This implements the proposed changes for LWG 3537 (which we're allowed
to do as an extension whatever the outcome of the issue). I noticed we
didn't implement LWG 2280 completely, as the std::begin and std::end
overloads for arrays were not noexcept.

libstdc++-v3/ChangeLog:

	* include/bits/range_access.h (begin(T (&)[N]), end(T (&)[N])):
	Add missing 'noexcept' as per LWG 2280.
	(rbegin(T (&)[N]), rend(T (&)[N]), rbegin(initializer_list<T>))
	(rend(initializer_list<T>)): Add 'noexcept' as per LWG 3537.
	* testsuite/24_iterators/range_access/range_access.cc: Check for
	expected noexcept specifiers. Check result types of generic
	std::begin and std::end overloads.
	* testsuite/24_iterators/range_access/range_access_cpp14.cc:
	Check for expected noexcept specifiers.
	* testsuite/24_iterators/range_access/range_access_cpp17.cc:
	Likewise.

(cherry picked from commit 00b46c00c8d9003c61e8f817668ad3380e16fedb)