queue<T, NonSpaceshipContainer> might be instantiated for a
non-three_way_comparable NonSpaceshipContainer. In that case, it
shouldn't attempt to grant friendship to the spaceship operator,
because that would attempt to form
compare_three_way_result_t<NonSpaceshipContainer>.

As far as I can tell, there's no way in the Core Language for a queue
to grant friendship to only one specialization of its non-member
spaceship operator while respecting the constraint. (See
WG21-N4861 [temp.friend]/9.) Instead, we need to template the
friendship declaration, such that queue grants friendship to all of its
spaceship operators. This is safe, just verbose.

We need a workaround until `std::pair` spaceship is implemented.

Reorder `smaller <=> smaller_equal` for consistency with
`smaller == smaller_equal` above.

This avoids anything potentially instantiating them before the
partial specialization vector<bool, Alloc> has been declared.

Co-authored-by: Casey Carter <[email protected]>

Testing `_Differing_bits == 0` might seem like it's asking the compiler
to do more work (always XORing). But it allows the compiler to see that
the input to `_Countr_zero` is nonzero, and that allows it to optimize
away the runtime ISA check, unconditionally emitting a `tzcnt`
instruction.

Then, at the end of the function, we know something that the compiler
doesn't realize: `_Mask` selects a differing bit (the least significant
one), so `(_Left & _Mask)` and `(_Right & _Mask)` are zero/nonzero
or vice versa. Therefore, we only need to inspect one side!

I've verified that each of these changes improves the codegen.

Co-authored-by: Casey Carter <[email protected]>

…o other changes).