[HLSL] Preserve coherence qualifiers on resource flat-conversions

[llvm-beanz]

PerformHLSLConversion forced the ICK_Flat_Conversion cast target to the structural/unqualified type, discarding the 'globallycoherent' (and 'reordercoherent') AttributedType. As a result, the converted expression's
type no longer reflected the coherence of its destination, forcing consumers
to reconstruct that information from declarations.

Re-apply the coherence AttributedType to the flat-conversion result when the
target is a resource type, so the converted expression's type carries the
coherence of its destination (e.g. a ResourceDescriptorHeap[] access used to
initialize a 'globallycoherent' resource).

Assisted by Claude Opus 4.8

[bob80905]

LGTM, but there are 2 probably basic questions I have.

1. Oddly in the github view, globallycoherent is highlighted as a keyword in red, but reordercoherent is not. Why is that?
2. ResourceDescriptorHeap isn't defined anywhere, yet the tests work. Is this a variable that exists implicitly in all shaders? I don't think I've seen this before.

[llvm-beanz]

LGTM, but there are 2 probably basic questions I have.

1. Oddly in the github view, globallycoherent is highlighted as a keyword in red, but reordercoherent is not. Why is that?

It is because the GitHub syntax plugin isn't up-to-date with SM 6.9. I think GitHub uses this format grammar, which hasn't been updated in 7 years...

2. ResourceDescriptorHeap isn't defined anywhere, yet the tests work. Is this a variable that exists implicitly in all shaders? I don't think I've seen this before.

ResourceDescriptorHeap and SamplerDescriptorHeap are part of SM 6.6's dynamic resources.