[ORC] Add more synchronization to TestLookupWithUnthreadedMaterialization.

Don't run tasks until their corresponding thread has been added to the running
threads vector. This is an extention to fda4300, which doesn't seem to have
been enough to fix the synchronization issues on its own.

Author: lhames

llvm/unittests/ExecutionEngine/Orc/CoreAPIsTest.cpp

@@ -1252,9 +1252,14 @@ TEST_F(CoreAPIsStandardTest, TestLookupWithThreadedMaterialization) {
   std::mutex WorkThreadsMutex;
   std::vector<std::thread> WorkThreads;
   ES.setDispatchTask([&](std::unique_ptr<Task> T) {
+    std::promise<void> WaitP;
     std::lock_guard<std::mutex> Lock(WorkThreadsMutex);
     WorkThreads.push_back(
-        std::thread([T = std::move(T)]() mutable { T->run(); }));
+        std::thread([T = std::move(T), WaitF = WaitP.get_future()]() mutable {
+          WaitF.get();
+          T->run();
+        }));
+    WaitP.set_value();
   });
 
   cantFail(JD.define(absoluteSymbols({{Foo, FooSym}})));