make progress_token::operator() const so it can be captured by lambdas (#673)

Author: oldnewthing

strings/base_coroutine_foundation.h

@@ -305,7 +305,7 @@ namespace winrt::impl
             return *this;
         }
 
-        void operator()(Progress const& result)
+        void operator()(Progress const& result) const
         {
             m_promise->set_progress(result);
         }

test/test/async_progress.cpp

@@ -19,8 +19,12 @@ namespace
     IAsyncOperationWithProgress<int, int> Operation(HANDLE event)
     {
         co_await resume_on_signal(event);
-        auto progress = co_await get_progress_token();
-        progress(123);
+
+        // Invoke from a lambda to ensure that operator() is const.
+        [progress = co_await get_progress_token()]()
+        {
+            progress(123);
+        }();
         co_return 1;
     }