Sema: Always add synthesized accessors in the same spot
We had some non-deterministic behavior where depending on
validation order, synthesized accessors would end up in
different places because we would sometimes just add them
at the end of the member list.
Now add the getter right after the storage, the setter
right after the getter and the materializeForSet right
after the setter.
This changes some test output where the declaration order
did not make sense before but should otherwise have no
functional effect.
(cherry picked from commit 38f27c04964f0ed4ce7da7701e499fce16c5c929)
