libstdc++: Use conditional noexcept in std::reverse_iterator [PR 94418]

libstdc++: Use conditional noexcept in std::reverse_iterator [PR 94418]

This adds a noexcept-specifier to each constructor and assignment
operator of std::reverse_iterator so that they are noexcept when the
corresponding operation on the underlying iterator is noexcept.

The std::reverse_iterator class template already requires that the
operations on the underlying type are valid, so we don't need to use the
std::is_nothrow_xxx traits to protect against errors when the expression
isn't even valid. We can just use a noexcept operator to test if the
expression can throw, without the overhead of redundantly checking if
the initialization/assignment would be valid.

Signed-off-by: Jonathan Wakely <jwakely@redhat.com>
libstdc++-v3/ChangeLog:

	PR libstdc++/94418
	* include/bits/stl_iterator.h (reverse_iterator): Use
	conditional noexcept on constructors and assignment operators.
	* testsuite/24_iterators/reverse_iterator/noexcept.cc: New test.