[clang] [HLSL][clang][Driver] Fix error when using the option -fcgl in --driver-mode=dxc. (PR #97001)

[Justin Bogner via cfe-commits]

https://github.com/bogner approved this pull request.

We may need to revisit the whole `ForwardedArguments` thing in general, as there are a few funny things going on looking at the `-cc1` options coming out of dxc - for example, before this change `-S` actually shows up twice sometimes.

In any case, this at least seems to make it less broken for now, so LGTM.

https://github.com/llvm/llvm-project/pull/97001