[clang] [clang-sycl-linker] AOT-compile split modules concurrently (PR #224548)

[Alexey Bader via cfe-commits]

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bader wrote:

> I'd prefer to do that in a follow-up PR, since this PR is a contained, low-risk change.

Sounds good to me.

https://github.com/llvm/llvm-project/pull/224548