LLVMType is both too broad and too narrow for defining DXIL operations, in
different ways. It's too broad in the sense that we don't need the full set of
MVTs - the set of types DXIL operations work on is much smaller. It's too
narrow in the sense that it's difficult to use it for the various fixed
structure types in DXIL, like `%dx.types.Handle` or `%dx.Types.ResRet.f32`.
Replace the usage of LLVMType in DXIL.td with DXILOpParamType, a simple class
that we can define an enum of types from. Further, use this to replace the
"ParameterKind" enum in DXILABI.h that has nothing to do with DXIL's ABI.
Pull Request: https://github.com/llvm/llvm-project/pull/104247
Add a mechanism to specify constraints to the design document. These facilitate
specification of DXIL Op attributes that are predicated by Shader Model version.
