[ASTReader] Add test for previous change r306583 / 145692e.

[graydon]

Summary

Add a test for the change to ASTReader that reproduces the
logic for consolidating multiple ObjC interface definitions to the
case of multiple ObjC protocol definitions.

This test is a modified copy of the test that accompanied the original
change to interfaces, in 2ba1979.

[bruno]

LGTM! Thanks Graydon

test/Modules/Inputs/lookup-assert-protocol/Base.h

+@protocol BaseProtocol
+- (void) test;
+@end

test/Modules/Inputs/lookup-assert-protocol/Derive.h

+#include "Base.h"
+@protocol DerivedProtocol<BaseProtocol>
+- (void) test2;
+@end

test/Modules/Inputs/lookup-assert-protocol/H3.h

+#include "Base.h"

test/Modules/Inputs/lookup-assert-protocol/module.map

+module X {
+  header "H3.h"
+  export *
+}

test/Modules/lookup-assert-protocol.m

+// RUN: rm -rf %t
+// RUN: %clang_cc1 -fmodules-cache-path=%t -fmodules -fimplicit-module-maps -I %S/Inputs/lookup-assert-protocol %s -verify
+// expected-no-diagnostics
+
+#include "Derive.h"
+#import <H3.h>
+
+__attribute__((objc_root_class))
+@interface Thing<DerivedProtocol>
+@end
+
+@implementation Thing
+- (void)test {
+}
+- (void)test2 {
+}
+@end