Add Half support in slang

Bug: 7342860

- Enable NativeHalfType and HalfArgsAndReturns for Langopts for API >=
- Add half and its vector variants to the list of reflected types
- Use addF16 as the FieldPacker method for setting Float16 values.
- Add tests that HalfArgumentsAndReturns is not enabled on API < 23.
  This should serve as an indirect test that NativeHalfType is also

Change-Id: I164ff6f8a3ed206326b904afa5430230b97d41c8
9 files changed