[SM 6.8] Reflection for Work Graph nodes plus more general DXIL Library reflection

[llvm-beanz]

The current D3D12LibraryReflection interfaces do not support exposing function attributes for node shaders. For example, if applications want to setup per-launch mode or per-node overrides they need to be able to identify the launch mode and/or node ID of each node.

Outstanding questions:

• Do we extend the existing library reflection interface to support retrieving those RDAT properties?
• Should we create a more general future-proof API that surfaces RDAT more directly?

Note: more details available in internal work item 94.

[damyanp]

Related: #6827

Current thinking is that we'll likely consider this as we bring up reflection for DXIL in clang.