chore: import upstream snapshot with attribution

tests/fixtures/objc_mixed/Bridging-Header.h

@@ -0,0 +1 @@
+#import "Widget.h"

tests/fixtures/objc_mixed/Widget.h

@@ -0,0 +1,4 @@
+@interface Widget
+- (void)render;
+- (void)refresh;
+@end

tests/fixtures/objc_mixed/Widget.m

@@ -0,0 +1,9 @@
+#import "Widget.h"
+
+@implementation Widget
+- (void)render {
+    [self refresh];
+}
+- (void)refresh {
+}
+@end

tests/fixtures/objc_mixed/WidgetExtras.swift

@@ -0,0 +1,5 @@
+extension Widget {
+    func describe() -> String {
+        return "widget"
+    }
+}