This adds an isolated component *IDxcDxrFallbackCompiler* that is used by the D3D12 Raytracing Fallback Compiler. The component is responsible for linking together DXR shaders into a state machine capable of emulating function invocations including recursion, patching HLSL intrinsics, export renaming, and emulating Shader Record support. This component is compiled to a DxrFallbackCompiler.dll that gets consumedd with D3D12 Raytracing Fallback Compiler apps.
For deeper implementation details, I will be fleshing out lib\DxrFallback\readme.md
Some leftover work still required:
1. Currently the Fallback Layer tests compile to an exe that must be manually run. This should be refactored to conform with existing DXIL unittests.
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