[clang] [llvm] [test] Avoid writing to a potentially write-protected dir (PR #89242)

[Fangrui Song via cfe-commits]

MaskRay wrote:

> but since we only care about stdout/stderr, throw away the output file and just write to /dev/null instead.

Since `-emit-llvm-only` is now used, the commit message should be updated. 

https://github.com/llvm/llvm-project/pull/89242