[BitwiseCopyable] Drop req from SIMD/Storage.
There are conformers to SIMDStorage (like that in the added test case)
which involve an Array (a type that can't conform to BitwiseCopyable).
So lift the constraint on SIMDStorage. This in turn requires lifting
the constraint on SIMD (otherwise, e.g. `SIMD8<Scalar>` would fail to
conform since it has as a member some SIMD8Storage which is only
constrained to conform to `SIMDStorage`; the `SIMD8Storage`
associatedtype also cannot be constrained to `BitwiseCopyable` because
that storage may again not conform as in the test example).
rdar://128661878
