[clang] [clang-sycl-linker] Make AOT temp files land relative to cwd, not -o's dir (PR #224506)

[Wenju He via cfe-commits]

wenju-he wrote:

> One thing that might be worth considering is parallel invocations with the same output basename but different output directories. With the basename-only approach, both could end up using the same intermediate .spv and .out files in the cwd.
> 
> Maybe it would be worth keeping some per-invocation uniqueness here while still using the output basename as the stem.

done in 32e8db1af6dbb7945f1372998200f53b2b00ad52, thanks

https://github.com/llvm/llvm-project/pull/224506