SILGen: Cope with overloads when emitting artificial main for @UIApplicationMain.

The overlay may add overloads for UIApplicationMain for various reasons. When we generate the special `main` implementation for a `@UIApplicationMain` class, we want to call the original function from Objective-C, so pick that one where there are multiple overloads to choose from. Fixes rdar://problem/42352695.

test/SILGen/Inputs/usr/include/UIKit.h

@@ -3,6 +3,12 @@
 @protocol UIApplicationDelegate
 @end
 
+#ifdef SILGEN_TEST_UIAPPLICATIONMAIN_NULLABILITY
+int UIApplicationMain(int argc, char *_Nullable *_Nonnull argv,
+                      NSString *_Nullable principalClassName, 
+                      NSString *_Nullable delegateClassName);
+#else
 int UIApplicationMain(int argc, char **argv,
                       NSString *principalClassName, 
                       NSString *delegateClassName);
+#endif