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

[David Blaikie via llvm-commits]

dwblaikie wrote:

Oh, maybe issues related to layered patches - this is intended to be submitted after the introduction of the ThreadPoolInterface? But includes those changes in this review at  the moment (I still haven't figured out what we're doing for dependent changes - and I thought the ThreadPoolInterface-introducing patch was already in - so not sure why it appears in this patch too)

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