This supports both built-in vector types and the SIMDX<T> Swift types
that are used through type aliases like float3. These SIMD types are
passed via compatibility structs (with their valued memcpy-d into the
ABI compatible struct) as they have special ABI rules. E.g., a float3 is
not passed as float3, but as float4 on the ABI level.
Previously, we did not expose simd types from Swift to C++ (unless the
SIMD types were impoted from clang).
rdar://153218744
