[HLSL] Add -Gis option from DXC to clang-dxc

[bob80905]

The dxc driver mode for clang, or clang-dxc.exe, needs to accept the -Gis compiler option, which dxc.exe accepts.
The -Gis compiler option enables IEEE strictness mode, constraining the behavior for NaNs and infs.
By enabling -Gis in clang-dxc, it will help to get more consistency across tests in the offload test suite that use these special float values for float16s.

[llvmbot]

@llvm/issue-subscribers-clang-driver

Author: Joshua Batista (bob80905)

The dxc driver mode for clang, or clang-dxc.exe, needs to accept the -Gis compiler option, which dxc.exe accepts. The -Gis compiler option enables IEEE strictness mode, constraining the behavior for NaNs and infs. By enabling -Gis in clang-dxc, it will help to get more consistency across tests in the offload test suite that use these special float values for float16s.