Deal with robustness for alloca / constant LUTs in DXIL

[HansKristian-Work]

Add some tests which explore various cases of UB.

• LUTs are generally robust, but can suffer from spill between LUTs. AMDVLK is not robust at all, but other drivers are. There's little cost in making this robust however, so just go for that.
• alloca() is robust on native drivers, and we are not in DXIL atm. DXBC needs it since games rely on it, and there are games that trip our expect-assume paths, so we should enable this by default to be safe.
• Groupshared behaves robustly across the board and our behavior matches native, so there's no point in going beyond that.
• Also refactors the option settings finally ...
• Bumps the bucket to 2.15 since this will replace a lot of shaders by default.

[HansKristian-Work]

For testing people, this requires a ton of spot checking across any DXIL based game you can come across. We're interested in game not exploding + any performance regressions you can find. Very minor (~1%) regressions are likely unavoidable. Progress can be reported on this PR for reference.

EDIT: When testing, make sure to disable Fossilize layer in Steam, since it's not certain we'll land this.