[mlir][async] Update comments about library registration. (NFC)

[ingomueller-net]

Summary

This updates the code comments about the library registration mechanism,
which changed in https://reviews.llvm.org/D153029, and which should have
updated as part of that patch.

[ingomueller-net]

This is a comment-only fix. I'll self-approve and land as soon as CI passes.

[ingomueller-net]

Comment-only change: self-accepting.

mlir/lib/ExecutionEngine/AsyncRuntime.cpp

 //===----------------------------------------------------------------------===//
 
 extern "C" void mlirAsyncRuntimePrintCurrentThreadId() {
   static thread_local std::thread::id thisId = std::this_thread::get_id();
   std::cout << "Current thread id: " << thisId << std::endl;
 }
 
 //===----------------------------------------------------------------------===//
-// MLIR Runner (JitRunner) dynamic library integration.
+// MLIR ExecutionEngine dynamic library integration.
 //===----------------------------------------------------------------------===//
 
-// Export symbols for the MLIR runner integration. All other symbols are hidden.
+// Export symbols for the MLIR ExecutionEngine integration. All other symbols
+// are hidden.
 #ifdef _WIN32
 #define API __declspec(dllexport)
 #else
 #define API __attribute__((visibility("default")))
 #endif
 
 // Visual Studio had a bug that fails to compile nested generic lambdas
 // inside an `extern "C"` function.