libstdc++: Make __normal_iterator constexpr, always_inline, nodiscard

The __gnu_cxx::__normal_iterator type we use for std::vector::iterator
is not specified by the standard, it's an implementation detail. This
means it's not constrained by the rule that forbids strengthening
constexpr. We can make it meet the constexpr iterator requirements for
older standards, not only when it's required to be for C++20.

For the non-const member functions they can't be constexpr in C++11, so
use _GLIBCXX14_CONSTEXPR for those. For all constructors, const members
and non-member operator overloads, use _GLIBCXX_CONSTEXPR or just
constexpr.

We can also liberally add [[nodiscard]] and [[gnu::always_inline]]
attributes to those functions.

Also change some internal helpers for std::move_iterator which can be
unconditionally constexpr and marked nodiscard.

libstdc++-v3/ChangeLog:

	* include/bits/stl_iterator.h (__normal_iterator): Make all
	members and overloaded operators constexpr before C++20, and add
	always_inline attribute
	(__to_address): Add nodiscard and always_inline attributes.
	(__make_move_if_noexcept_iterator): Add nodiscard
	and make unconditionally constexpr.
	(__niter_base(__normal_iterator), __niter_base(Iter)):
	Add nodiscard and always_inline attributes.
	(__niter_base(reverse_iterator), __niter_base(move_iterator))
	(__miter_base): Add inline.
	(__niter_wrap(From, To)): Add nodiscard attribute.
	(__niter_wrap(const Iter&, Iter)): Add nodiscard and
	always_inline attributes.

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