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

[Mehdi Amini via cfe-commits]

joker-eph wrote:

Actually no: the first patch landed already, so `ThreadPoolInterface` is now the base class in the codebase. I have some mixup here in that when renaming ThreadPool -> DefaultThreadPool, I used the base class ThreadPoolInterface when updating some of the uses.

I will push these ahead as a NFC change and rebase, that'll reduce the diff here.

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