Rename ThreadPool to StdThreadPool

`ThreadPool` was split up in LLVM
6594f428de91e333c1cbea4f55e79b18d31024c4.

Author: bnbarham

include/swift/DependencyScan/ModuleDependencyScanner.h

@@ -145,7 +145,7 @@ class ModuleDependencyScanner {
   /// The available pool of workers for filesystem module search
   unsigned NumThreads;
   std::list<std::unique_ptr<ModuleDependencyScanningWorker>> Workers;
-  llvm::ThreadPool ScanningThreadPool;
+  llvm::StdThreadPool ScanningThreadPool;
   /// Protect worker access.
   std::mutex WorkersLock;
 };

tools/swift-scan-test/swift-scan-test.cpp

@@ -221,7 +221,7 @@ int main(int argc, char *argv[]) {
   auto Args = createArgs(SwiftCommands, Saver);
 
   std::atomic<int> Ret = 0;
-  llvm::ThreadPool Pool(llvm::hardware_concurrency(Threads));
+  llvm::StdThreadPool Pool(llvm::hardware_concurrency(Threads));
   for (unsigned i = 0; i < Threads; ++i) {
     Pool.async([&]() {
       switch (Action) {