[ios18 support] add new case for Metal Version 3.2 that is on ios 18 -- reroutes MoltenMK to use version 3.1

[Daij-Djan]

This adds a new preprocessor macro for xcode16 / ios18 sdk and reroutes the version to 3.1 (additions to MVKDevice are iline with as what we did for other xcode versions in the past)

[CLAassistant]

All committers have signed the CLA.

[billhollings]

Thanks for taking care of this, and for submitting this PR!

reroutes the version to 3.1
...
// We don't explicitly support Metal 3.2 yet

What do you mean by this? Is there a reason why you are not using MTLLanguageVersion3_2? The MVK_XCODE_16 and mvkOSVersionIsAtLeast() guard code should protect against inappropriate build or runtime usage. Have you tried building or running, and are experiencing problems?

In addition, can you also add the equivalent changes to OSSupport.mm?

Thanks.

[Daij-Djan]

I didnt wanna make any logic changes so I just made it 3.1 -- I didnt test runtime behaviour of moltenVK when I did the change.

[billhollings]

Change it to MTLLanguageVersion3_2 and test whether you can build using Xcode 15, to make sure it doesn't cause a regression break. If that is successful, we should be good.

If you have Xcode 16 up and running, test a build under that as well. If not, it should be good enough to go ahead with MTLLanguageVersion3_2 anyway.