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

[David Blaikie via llvm-commits]

dwblaikie wrote:

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

One patch `ThreadPool->DefaultThreadPool` (people get a build error about `ThreadPool` not being the name of anything, find this patch as the root cause, and rename all their ThreadPool->DefaultThreadPool)
Follow-up patch, `ThreadPoolInterface->ThreadPool` (similarly clear/separate errors)

Changing both in one patch risks folks getting confusing error messages because their existing ThreadPool usage will now instantly start being interpreted as the interface type - resulting in different/confusing errors about inability to instantiate abstract types, etc, presumably. Rather than just that the name is no longer present at all.

Ultimately they can root cause and figure out both renamings - probably not a huge deal either way, but my understanding was that separating them might be marginally better for downstrteamers.

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