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
diff --git a/MachO/Writer.cpp b/MachO/Writer.cpp
index 65b598d..9b0a32c 100644
--- a/MachO/Writer.cpp
+++ b/MachO/Writer.cpp
@@ -66,7 +66,7 @@
 
   template <class LP> void run();
 
-  ThreadPool threadPool;
+  DefaultThreadPool threadPool;
   std::unique_ptr<FileOutputBuffer> &buffer;
   uint64_t addr = 0;
   uint64_t fileOff = 0;