[cxx-interop] Update the std::pair safety test

The test was passing by accident: we didn't properly check `std::pair` type for completeness, and assumed that it is incomplete.

This updates the test to actually verify the behavior that is specified in the vision document.

Author: egorzhdan

test/Interop/Cxx/stdlib/Inputs/std-pair.h

@@ -35,7 +35,7 @@ struct __attribute__((swift_attr("import_iterator"))) Iterator {};
 using PairUnsafeStructInt = std::pair<UnsafeStruct, int>;
 using PairIteratorInt = std::pair<Iterator, int>;
 
-struct HasMethodThatReturnsUnsafePair {
+struct __attribute__((swift_attr("import_owned"))) HasMethodThatReturnsUnsafePair {
     PairUnsafeStructInt getUnsafePair() const { return {}; }
     PairIteratorInt getIteratorPair() const { return {}; }
 };

test/Interop/Cxx/stdlib/use-std-pair-typechecker.swift

@@ -1,7 +1,12 @@
-// RUN: %target-typecheck-verify-swift -I %S/Inputs -enable-experimental-cxx-interop
+// RUN: not %target-swift-frontend %s -typecheck -I %S/Inputs -enable-experimental-cxx-interop 2>&1 | %FileCheck %s
 
 import StdPair
 
 let u = HasMethodThatReturnsUnsafePair()
-u.getUnsafePair() // expected-error {{value of type 'HasMethodThatReturnsUnsafePair' has no member 'getUnsafePair'}}
-u.getIteratorPair() // expected-error {{value of type 'HasMethodThatReturnsUnsafePair' has no member 'getIteratorPair'}}
+u.getUnsafePair()
+// CHECK: error: value of type 'HasMethodThatReturnsUnsafePair' has no member 'getUnsafePair'
+// CHECK: note: C++ method 'getUnsafePair' may return an interior pointer
+
+u.getIteratorPair()
+// CHECK: error: value of type 'HasMethodThatReturnsUnsafePair' has no member 'getIteratorPair'
+// CHECK: note: C++ method 'getIteratorPair' may return an interior pointer