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

[Wenju He via cfe-commits]

wenju-he wrote:

The merge commit is due to a2fdb8fe0d09dbc1d543c3381f0ddacdf7054c5e and change in the merge commit should be just an nfc refactor to adapt to a2fdb8fe0d09dbc1d543c3381f0ddacdf7054c5e

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