[HLSL] Correctly set `__HLSL_ENABLE_16_BIT` (#89788)

The preprocessor define `__HLSL_ENABLE_16_BIT` should be set to 1 if
native 16-bit types are enabled and not set if they are not.

Previously we were setting the value to match the HLSL active language
version, and we had no test coverage verifing the value was set and not
set as expected.

Fixes #89787