[clang] [clang-tools-extra] [lld] [lldb] [llvm] [mlir] Rename llvm::ThreadPool -> llvm::DefaultThreadPool (NFC) (PR #83702)

[Mehdi Amini via cfe-commits]

joker-eph wrote:

I did the first part of the renaming @dwblaikie : looks good?

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