Enable "-HV 2021" option for DXC

[jkwak-work]

Closes #6442

[jkwak-work]

This PR cannot be merged until I resolve the duplicated witness table problem.
CI error shows the same problem as when I tried to upgrade DXC 1.7 to DXC 1.9.

    dxc 1.7: C:/actions-runner/_work/slang/slang/FalcorBin/build/windows-vs2022/bin/Release/shaders/Utils/Math/ShadingFrame.slang(116): error :  no matching function for call to 's_fwd_d_dot_0'
    dxc 1.7: note :     DiffPair_6 _S915 = s_fwd_d_dot_0(_S913, _S914);
    dxc 1.7: note :                        ^~~~~~~~~~~~~
    dxc 1.7: C:/actions-runner/_work/slang/slang/FalcorBin/build/windows-vs2022/bin/Release/shaders/DiffRendering/DiffSceneQuery.slang(239): error :  candidate function not viable: no known conversion from 'DiffPair_4' to 'DiffPair_10' for 1st argument
    dxc 1.7: note : DiffPair_6 s_fwd_d_dot_0(DiffPair_10 dpdpx_3, DiffPair_10 dpdpy_0)
    dxc 1.7: note :            ^
    dxc 1.7: C:/actions-runner/_work/slang/slang/FalcorBin/build/windows-vs2022/bin/Release/shaders/Utils/Math/ShadingFrame.slang(117): error :  no matching function for call to 's_fwd_d_cross_0'
    dxc 1.7: note :     DiffPair_10 _S927 = s_fwd_d_cross_0(_S914, _S926);
    dxc 1.7: note :                         ^~~~~~~~~~~~~~~