Tests: Add a test for rdar://158172056.

The bug tracked by rdar://158172056 was already fixed by
https://github.com/swiftlang/swift/pull/83607 but this additional test case is
needed to ensure that conformances to Obj-C protocols with obsolete
requirements imported under legacy spellings not regress again.

test/Inputs/clang-importer-sdk/usr/include/Foundation.h

@@ -1243,3 +1243,11 @@ void takeNullableId(_Nullable id);
 @interface PaletteDescriptor : NSObject <NSCopying>
 @property (readonly, nonnull) ColorArray *colors;
 @end
+
+@protocol NSIndexable <NSObject>
+- (id)objectAtIndex:(NSInteger)index;
+@end
+
+@interface NSCouldConformToIndexable : NSObject
+- (id)objectAtIndex:(NSInteger)index;
+@end