[GR-50240] Wrap HotSpotObjectConstant directly in ImageHeapConstant.

[graalvmbot]

This PR removes the SubstrateObjectConstant representation for hosted objects. Now ImageHeapConstant wraps the HotSpotObjectConstant directly. It removes AnalysisUniverse.toHosted() as there's no need to convert a hosted SubstrateObjectConstant into a HotSpotObjectConstant. Parts of AnalysisUniverse.fromHosted() are still kept as we still want the word types and simulated ImageHeapConstant interception. That logic now lives in SVMHostedValueProvider.interceptHosted().

We still transform between the ImageHeapConstant and SubstrateObjectConstant when we encode/decode graphs during image building. See SubstrateGraalUtils.forRuntime()/SubstrateGraalUtils.forHosted(). At some point we could try to simply encode the graphs using the ImageHeapConstant during image building and just transform to SubstrateObjectConstant when we write out the heap.

This PR also adds a proper IdentityHashCodeProvider mechanism to allow custom manipulation and reuse of identity hash codes during image build in the context of substituted types, e.g., DynamicHub.

See individual commits:

• Clean-up unnecessary fallback when a constant is not ImageHeapConstant.
• RemoveSubstrateObjectConstant from ImageHeapConstant.
• Use SubstrateObjectConstant for runtime compiled graphs only.
• HostedSnippetReflectionProvider can implement SnippetReflectionProvider directly.
• Refactor hosted value interception.