[MLIR] Fix compilation with LLVM_ENABLE_THREADS=OFF

[jdd]

Summary

Currently, compiles with LLVM_ENABLE_THREADS=OFF fail due to this symbol missing. Add it but assert as calling code is (and should be) checking that threading is enabled.

[dexonsmith]

llvm/lib/Support/ThreadPool.cpp

121

Perhaps llvm::report_fatal_error() would be appropriate here.

[MaskRay]

llvm/lib/Support/ThreadPool.cpp

121

There was a -Wreturn-type warning in -DLLVM_ENABLE_THREADS=OFF -DLLVM_ENABLE_ASSERTIONS=off build.

Fixed by 27cb5eea00a8841323f16d5914d9dec29eb03ef7

llvm/lib/Support/ThreadPool.cpp

void ThreadPool::wait() {
   // Sequential implementation running the tasks
   while (!Tasks.empty()) {
     auto Task = std::move(Tasks.front());
     Tasks.pop();
     Task();
   }
 }
 
+bool ThreadPool::isWorkerThread() const {
+  assert(false && "LLVM compiled with threading disabled");
+}
+
 ThreadPool::~ThreadPool() { wait(); }
 
 #endif