[SPIR-V] Add support for D3D12 descriptor heaps

[Keenuts]

This commit adds support for Sampler/Resource descriptor heaps in DXC. Support for those heaps on the SPIR-V side requires no other extension than SPV_EXT_descriptor_indexing.
On the Vulkan side, the VK_EXT_mutable_descriptor_type will be required as multiple descriptor types must be allowed on the same binding.

When loading a type from a heap, DXC generates a new OpRuntimeArray of the correct type, and binds it to set=0, binding=<BindingNumberOfTheHeap>.
This means multiple OpRuntimeArrays will share the same binding. This is why VK_EXT_mutable_descriptor_type is required.

This implementation uses at most 3 bindings:

• N OpRuntimeArray as binding A for the ResourceDescriptorHeap
• N OpRuntimeArray as binding B for the SamplerDescriptorHeap
• 1 OpRuntimeArray %counter_type for the ResourceDescriptorHeap counters.

The bindings are only allocated if used. If only the SamplerDescriptorHeap is used, a single binding is required.

The binding allocation logic is:

1. allocate bindings for every resources, excluding heaps.
2. If ResourceDescriptorHeap is used, find the first unused binding in set=0 and use it.
3. Same for the SamplerDescriptorHeap
4. Same for the counters.

UAV counters are not always created, only if used.
When used, they are stored in an OpRuntimeArray. The index of a counter in that array
is equal to the index of the associated resource in its own OpRuntimeArray.

        RWStructuredBuffer a = ResourceDescriptorHeap[2];
        a.IncrementCounter();
        // buffer in descriptorSet 0, binding 0, OpRuntimeArray[index=2]
        // counter in descriptorSet 0, binding 1, OpRuntimeArray[index=2]

As-is, this PR doesn't allow resource heaps to alias regular resources, or to overlap.
A follow-up PR will add 3 flags to override each binding/set pairs:

• 'fvk-bind-resource-heap '
• 'fvk-bind-sampler-heap '
• 'fvk-bind-counter-heap '

[s-perron]

The implementation looks pretty good. But we need to work on how the bindings and descriptor sets are assigned. There does seem to be some misunderstanding. We also need to consider the interaction with -fspv-flatten-resource-arrays.

[Keenuts]

Resolving comments around bindings as the design changed since the last version.
Binding logic to implement now:

• All other resources follow the same rules as today (implicit & explicit bindings)
• Then, we find the 3 first unused bindings (not necessarily consecutive, starting at set 0, binding 0)
• Take 1 binding for resource heaps, 1 for sample heaps, and 1 for UAV counters.
• index in the binding for UAV counter matches the index of the associated UAV in the resource heap binding.
• 3 flags will be added: -fvk-bind-[resource,sampler,counter]-heap <set> <binding>, which allow the user to fix the binding/set pair, allowing aliasing resources.

[Keenuts]

@s-perron I think this is ready for another review. I modified the PR description and the documentation to match the new binding allocation behavior.
This PR does not include the new flags as I find it already quite big.

[s-perron]

Looks good.

I have one more case to consider. I expect we would want to disallow passing the heaps as function parameters. As far as I know, it cannot be done without templates because we do not even know the type.

[Keenuts]

Added the error + test, and rebased on main.

[sudonatalie]

LGTM!