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

[Joseph Huber via cfe-commits]

jhuber6 wrote:

I'll need to look into whether or not it's possible to make `-Xarch_spirv64 -flto` work.

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