utils: enable swift_Concurrency on the new build

Enable the Concurrency runtime for the experimental runtime builds on
Windows. This is required to enable the use of the static runtime to
bootstrap the early swift-driver.

Author: compnerd

utils/build.ps1

@@ -2302,6 +2302,7 @@ function Build-ExperimentalRuntime {
         CMAKE_STATIC_LIBRARY_PREFIX_Swift = "lib";
         CMAKE_SYSTEM_NAME = $Platform.OS.ToString();
         dispatch_DIR = (Get-ProjectCMakeModules $Platform Dispatch);
+        SwiftCore_ENABLE_CONCURRENCY = "YES";
       }
   }
 }