[mlir] Fix mlir_async_runtime.lib creation for MSVC.

[jamboree]

Summary

The extern "C" functions exported with MLIR_ASYNCRUNTIME_EXPORT are inside namespace mlir::runtime. MSVC has problems in associating the decls with the impls if they are not qualified properly.

This patch put the impls inside the mlir::runtime namespace as the decls.

[ezhulenev]

mlir/lib/ExecutionEngine/AsyncRuntime.cpp

174

Maybe add a comment that it violates https://llvm.org/docs/CodingStandards.html#use-namespace-qualifiers-to-implement-previously-declared-functions for a reason?

[jamboree]

Add a comment as requested.

mlir/lib/ExecutionEngine/AsyncRuntime.cpp

 #include <cassert>
 #include <condition_variable>
 #include <functional>
 #include <iostream>
 #include <mutex>
 #include <thread>
 #include <vector>
 
-using namespace mlir::runtime;
-
 //===----------------------------------------------------------------------===//
 // Async runtime API.
 //===----------------------------------------------------------------------===//
 
 namespace mlir {
 namespace runtime {
 namespace {
 
struct AsyncGroup : public RefCounted {
 
   // Internal state below guarded by a mutex.
   std::mutex mu;
   std::condition_variable cv;
 
   std::vector<std::function<void()>> awaiters;
 };
 
-} // namespace runtime
-} // namespace mlir
+//===----------------------------------------------------------------------===//
+// These extern "C" functions have their prototypes declared in C++
+// namespace, to have them properly exported by MSVC, they need to be inside the
+// same namespace or be qualified with the namespace.
+//===----------------------------------------------------------------------===//
 
 // Adds references to reference counted runtime object.
 extern "C" void mlirAsyncRuntimeAddRef(RefCountedObjPtr ptr, int32_t count) {
   RefCounted *refCounted = static_cast<RefCounted *>(ptr);
   refCounted->addRef(count);
 }
 
 // Drops references from reference counted runtime object.
 extern "C" void mlirAsyncRuntimeDropRef(RefCountedObjPtr ptr, int32_t count) {
   RefCounted *refCounted = static_cast<RefCounted *>(ptr);

 // Small async runtime support library for testing.
 //===----------------------------------------------------------------------===//
 
 extern "C" void mlirAsyncRuntimePrintCurrentThreadId() {
   static thread_local std::thread::id thisId = std::this_thread::get_id();
   std::cout << "Current thread id: " << thisId << std::endl;
 }
 
+} // namespace runtime
+} // namespace mlir
+
 #endif // MLIR_ASYNCRUNTIME_DEFINE_FUNCTIONS