[MLIR] Fix compilation with LLVM_ENABLE_THREADS=OFF

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.

Differential Revision: https://reviews.llvm.org/D116846

llvm/lib/Support/ThreadPool.cpp

@@ -117,6 +117,10 @@ void ThreadPool::wait() {
   }
 }
 
+bool ThreadPool::isWorkerThread() const {
+  assert(false && "LLVM compiled with threading disabled");
+}
+
 ThreadPool::~ThreadPool() { wait(); }
 
 #endif