Add Interpreter test for @_objcImpl

Author: beccadax

test/Interpreter/Inputs/objc_implementation.h

@@ -0,0 +1,7 @@
+#import <Foundation/Foundation.h>
+
+@interface ImplClass : NSObject
+
+- (nonnull NSString *)someMethod;
+
+@end

test/Interpreter/objc_implementation.swift

@@ -0,0 +1,12 @@
+// RUN: %target-run-simple-swift(-import-objc-header %S/Inputs/objc_implementation.h) %s | %FileCheck %s
+// REQUIRES: executable_test
+// REQUIRES: objc_interop
+
+import Foundation
+
+@_objcImplementation extension ImplClass {
+  @objc func someMethod() -> String { "ImplClass.someMethod()" }
+}
+
+print(ImplClass().someMethod())
+// CHECK: ImplClass.someMethod()