Use instancetype for the benchmark and SwiftNativeNSObject

This is more for consistency than anything else, really.

benchmark/utils/ObjectiveCTests/ObjectiveCTests.h

@@ -23,7 +23,7 @@ NS_ASSUME_NONNULL_BEGIN
   NSArray<NSString *> *bridgedStrings;
 }
 
-- (id)init;
+- (instancetype)init NS_DESIGNATED_INITIALIZER;
 - (void)setUpStringTests:(NSArray<NSString *> *)bridgedStrings;
 - (void)testFromString:(NSString *) str;
 - (NSString *)testToString;

benchmark/utils/ObjectiveCTests/ObjectiveCTests.m

@@ -102,7 +102,7 @@
 
 @implementation BridgeTester
 
-- (id)init {
+- (instancetype)init {
   self = [super init];
   if (!self)
     return self;