MSL: Remove special case for threadgroup array wrapper.

[HansKristian-Work]

I cannot find any evidence that this does not actually work. The original case here was from Epic's PR series in 2019, but I cannot see why it doesn't work. It might have been a bug in a very old compiler at some point.

Fix #2239.

[HansKristian-Work]

@cdavis5e @LukasBanana Please check if this PR breaks anything for you.

[LukasBanana]

I don't see a problem with this PR since it's handling a case where a wrapper is not allowed. I haven't seen this wrapper in a while but also haven't been working with the Metal backend in our shader pipeline for quite some time. iirc. the idea was to workaround the issue of copying arrays locally but I don't remember the case where this happened off the top of my head. I wouldn't mind if we can get rid of it entirely, but for now, this change should be good.

[HansKristian-Work]

it's handling a case where a wrapper is not allowed

Not sure what you mean, I'm considering that more cases are allowed here.

The original commit which made this claim was:

commit d50659af92fc379ba33e26cf5c39d4233396cb38
Author: Mark Satterthwaite <[email protected]>
Date:   Tue Aug 13 18:18:48 2019 -0400

    Rework the way arrays are handled in Metal to remove the array copies as they are unnecessary from Metal 1.2. There were cases where copies were not being inserted and others appeared unncessary, using the template type should allow the 'metal' compiler to do the best possible optimisation. The changes are broken into three stages.

1. Allow Metal to use the array<T> template to make arrays a value type.
2. Force the use of C style array declaration for some cases which cannot be wrapped with a template.
3. Threadgroup arrays can't have a wrapper type.
4. Tweak the code to use unsafe_array in a few more places so that we can handle passing arrays of resources into the shader and then through shaders into sub-functions.
5. Handle packed matrix types inside arrays within structs.
6. Make sure that builtin arguments still retain their array qualifiers when used in leaf functions.
7. Fix declaration of array-of-array constants for Metal so we can use the array<T> template.

3. Threadgroup arrays can't have a wrapper type. is wrong I think and is causing me immense pain at the moment since there are various bugs that are unreasonably difficult to fix.

[HansKristian-Work]

Gonna go ahead and merge this. If this somehow breaks, it can always be reverted.

[LukasBanana]

Sorry for the late response.

Not sure what you mean, I'm considering that more cases are allowed here.

I meant to say you were probably right, but didn't formulate that well. Perhaps threadgroup arrays were more problematic in earlier versions of Metal. UE5 minimum Metal language support is 2.2 for macOS and 2.4 for iOS.