Use a per-thread VerifierDeps.

Avoid lock contention on a singleton VerifierDeps by allocating
temporary per-thread VerifierDeps that get merged after verification.

This saves around ~35% compile-times on interpret-only.

Only the creation of extra strings is guarded by a lock, for simplicity.

Test: test-art-host, test-art-target
bug: 32641252
bug: 30937355

Change-Id: I11a2367da882b58e39afa7b42cba2e74a209b75d
diff --git a/runtime/thread_pool.cc b/runtime/thread_pool.cc
index 65fd999..d9d2ea3 100644
--- a/runtime/thread_pool.cc
+++ b/runtime/thread_pool.cc
@@ -85,6 +85,7 @@
   ThreadPoolWorker* worker = reinterpret_cast<ThreadPoolWorker*>(arg);
   Runtime* runtime = Runtime::Current();
   CHECK(runtime->AttachCurrentThread(worker->name_.c_str(), true, nullptr, false));
+  worker->thread_ = Thread::Current();
   // Do work until its time to shut down.
   worker->Run();
   runtime->DetachCurrentThread();