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

[Manuel Carrasco via cfe-commits]

mgcarrasco wrote:

Thanks. I did a quick test with [this example](https://github.com/llvm/llvm-project/pull/201457#issuecomment-4670541098), and this one still crashes:

`clang++ -x hip hello.hip --offload-arch=amdgcnspirv -use-spirv-backend --no-offload-new-driver`

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