[clang] [AMDGPU][SPIR-V] Fix treating SPIR-V input as the wrong LLVM-IR (PR #202986)

[Joseph Huber via cfe-commits]

jhuber6 wrote:

> clang++: warning: argument unused during compilation: '-use-spirv-backend' [-Wunused-command-line-argument]

That's a weird flag, normally options like that are supposed to be `--` not `-`. Anyways, do you know if this is actually unused?

https://github.com/llvm/llvm-project/pull/202986