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

[Manuel Carrasco via cfe-commits]

mgcarrasco wrote:

Thanks @jhuber6, it fixed the remaining warning issue. The PR LGTM but please wait for @MrSidims's approval.

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