Rename llvm::ThreadPool -> llvm::DefaultThreadPool (NFC) (#83702)

The base class llvm::ThreadPoolInterface will be renamed
llvm::ThreadPool in a subsequent commit.

This is a breaking change: clients who use to create a ThreadPool must
now create a DefaultThreadPool instead.

GitOrigin-RevId: 716042a63f26cd020eb72960f72fa97b9a197382
2 files changed