[clang] [HLSL] Add `-Gis` option to clang-dxc (PR #146448)

[Chris B via cfe-commits]

https://github.com/llvm-beanz approved this pull request.

This will have some nuanced differences from what DXC does today because of the differences in granularity of floating point semantic controls in LLVM today vs 3.7. That said, I think this will be conservatively correct and is probably the best approach.

https://github.com/llvm/llvm-project/pull/146448