Add a new langopt NativeInt16Type to control support for 16 bit integers. Enable by default for all languages but HLSL. HLSL defines uint16_t and int16_t as a typedef of short. If -enable-16bit-types is not used, the typedefs don't exist so int16_t and uint16_t can't be used. However, short was still allowed. This change will produce an error 'unknown type name short' if -enable-16bit-types isn't used. Update failing tests. Add new test. Closes #81779