DispatchStubs: make more Windows friendly

Provide an alias for the import symbol as the ObjC runtime is normally
in a separate DLL and the compiler will annotate the function as being
DLLImport.  This allows us to resolve the symbols when building the SDK
overlay.

Signed-off-by: Kim Topley <[email protected]>

Author: compnerd

src/swift/DispatchStubs.cc

@@ -67,7 +67,11 @@ extern "C" void * objc_retainAutoreleasedReturnValue(void *obj);
 // eventually call swift_release to balance the retain below. This is a
 // workaround until the compiler no longer emits this callout on non-ObjC
 // platforms.
-extern "C" void swift_retain(void *);
+extern "C"
+#if defined(_WIN32)
+__declspec(dllimport)
+#endif
+void swift_retain(void *);
 
 DISPATCH_RUNTIME_STDLIB_INTERFACE
 extern "C" void * objc_retainAutoreleasedReturnValue(void *obj) {
@@ -78,4 +82,9 @@ extern "C" void * objc_retainAutoreleasedReturnValue(void *obj) {
     else return NULL;
 }
 
+#if defined(_WIN32)
+extern "C" void *(*__imp_objc_retainAutoreleasedReturnValue)(void *) =
+    &objc_retainAutoreleasedReturnValue;
+#endif
+
 #endif // !USE_OBJC