[libcxx] <experimental/simd> Added internal storage type, constructors,...

[libcxx] <experimental/simd> Added internal storage type, constructors, subscript operators of class simd/simd_mask and related tests

[libcxx] <experimental/simd> Added internal storage type for class simd/simd_mask
[libcxx] <experimental/simd> Added all constructors of class simd/simd_mask and related tests
[libcxx] <experimental/simd> Added basic simd reference implementation, subscript operators of class simd/simd_mask and related tests

Reviewed By: #libc, philnik

Differential Revision: https://reviews.llvm.org/D144364