Fix bindless resource heap placed in wrong descriptor set

[gtong-nv]

When a ParameterBlock contains only resources(no uniform data), e.g.

struct Texture2DHandleBuffer
{
    StructuredBuffer<Texture2D.Handle> texture2DbufferOfHandles;
};

ParameterBlock<Texture2DHandleBuffer> BLOCK;

its elements get flattened into global params with RegisterSpace offsets.
The findUnusedSpaceIndex() function was not tracking these RegisterSpace
offsets, causing __slang_resource_heap to be incorrectly placed in the
same descriptor set as the ParameterBlock's resources.

Add tracking of RegisterSpace offsets directly (as absolute descriptor
set numbers) to ensure the bindless resource heap is placed in an
unused descriptor set.

#8952

[tdavidovicNV]

I've tested this branch and can confirm it solves the bug even on the original code from which I did the simplified repro case.

[gtong-nv]

I've tested this branch and can confirm it solves the bug even on the original code from which I did the simplified repro case.

Thanks for confriming. I will request a review again and try to get this merged today

[jkwak-work]

Looks good to me.

But I wonder if the test should check the reflection info as well.

[gtong-nv]

Yong gave some feedback during the review meeting. I will refactor it a bit and update

[CLAassistant]

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