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

[Dmitry Sidorov via cfe-commits]

MrSidims wrote:

> This is piling hacks on hacks but could you make a PR for it?

Sure, will push it shortly.

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