[lld] [lldb] [llvm] [mlir] Rename ThreadPool->DefaultThreadPool and ThreadPoolInterface->ThreadPool (NFC) (PR #83702)

[Mehdi Amini via llvm-commits]

joker-eph wrote:

@dwblaikie : how would you split it? I didn't quite get the two renamings you have in mind?

https://github.com/llvm/llvm-project/pull/83702