libstdc++: Inline memmove optimizations for std::copy etc. [PR115444]

This removes all the __copy_move class template specializations that
decide how to optimize std::copy and std::copy_n. We can inline those
optimizations into the algorithms, using if-constexpr (and macros for
C++98 compatibility) and remove the code dispatching to the various
class template specializations.

Doing this means we implement the optimization directly for std::copy_n
instead of deferring to std::copy, That avoids the unwanted consequence
of advancing the iterator in copy_n only to take the difference later to
get back to the length that we already had in copy_n originally (as
described in PR 115444).

With the new flattened implementations, we can also lower contiguous
iterators to pointers in std::copy/std::copy_n/std::copy_backwards, so
that they benefit from the same memmove optimizations as pointers.
There's a subtlety though: contiguous iterators can potentially throw
exceptions to exit the algorithm early.  So we can only transform the
loop to memmove if dereferencing the iterator is noexcept. We don't
check that incrementing the iterator is noexcept because we advance the
contiguous iterators before using memmove, so that if incrementing would
throw, that happens first. I am writing a proposal (P3349R0) which would
make this unnecessary, so I hope we can drop the nothrow requirements
later.

This change also solves PR 114817 by checking is_trivially_assignable
before optimizing copy/copy_n etc. to memmove. It's not enough to check
that the types are trivially copyable (a precondition for using memmove
at all), we also need to check that the specific assignment that would
be performed by the algorithm is also trivial. Replacing a non-trivial
assignment with memmove would be observable, so not allowed.

libstdc++-v3/ChangeLog:

	PR libstdc++/115444
	PR libstdc++/114817
	* include/bits/stl_algo.h (__copy_n): Remove generic overload
	and overload for random access iterators.
	(copy_n): Inline generic version of __copy_n here. Do not defer
	to std::copy for random access iterators.
	* include/bits/stl_algobase.h (__copy_move): Remove.
	(__nothrow_contiguous_iterator, __memcpyable_iterators): New
	concepts.
	(__assign_one, _GLIBCXX_TO_ADDR, _GLIBCXX_ADVANCE): New helpers.
	(__copy_move_a2): Inline __copy_move logic and conditional
	memmove optimization into the most generic overload.
	(__copy_n_a): Likewise.
	(__copy_move_backward): Remove.
	(__copy_move_backward_a2): Inline __copy_move_backward logic and
	memmove optimization into the most generic overload.
	* testsuite/20_util/specialized_algorithms/uninitialized_copy/114817.cc:
	New test.
	* testsuite/20_util/specialized_algorithms/uninitialized_copy_n/114817.cc:
	New test.
	* testsuite/25_algorithms/copy/114817.cc: New test.
	* testsuite/25_algorithms/copy/115444.cc: New test.
	* testsuite/25_algorithms/copy_n/114817.cc: New test.

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

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