[clang] [SPIRV] Add -spirv option to DXC driver (PR #65989)

[Natalie Chouinard via cfe-commits]

sudonatalie wrote:

@llvm-beanz Can you take a look at this PR? I don't seem to be able to assign Reviewers, maybe because I don't have committer status yet.

https://github.com/llvm/llvm-project/pull/65989