cpp/experimental/simd/simd mask/simd mask

@1@ Constructs a using  (constructed without initializer) or  (constructed with an empty initializer). @2@ The broadcast constructor constructs a with all values initialized to. @3@ Constructs a where the i-th element is initialized to. . @4@ The load constructor constructs a where the i-th element is initialized to. @5-6@ Implicitly declared copy and move constructors. Constructs a where each element is initialized from the values of the elements in.