[DirectX] Missing verifications in `validate` of `DXILRootSignature.cpp`

[inbelic]

This issue tracks resolving some discrepancies in verification during validate in DXILRootSignature.cpp.

Namely,

• verifyDescriptorFlag and verifyDescriptorRangeFlag should be updated to reflect [Root Signatures] Fix default values for Descriptor Range Flags and Root Descriptor Flags in Version 1 wg-hlsl#297
• There is currently no verification that numDescriptors > 0 which should be added to be compliant with DXC: https://github.com/microsoft/DirectXShaderCompiler/blob/4fcf67f78f7d6ffd286316112694a3ae000860e2/lib/DxilRootSignature/DxilRootSignatureValidator.cpp#L197.

AC:

• Update verifyDescriptorFlag and verifyDescriptorRangeFlag such that they correctly reflect the validations documented
• Add verifyNumDescriptors to the validation of DescriptorRanges