[cxx-interop] Update tests for mutable struct fields

Methods that are marked as `const` in C++ can still modify a field of the object if the field is marked as `mutable`.

We previously imported all methods of structs with mutable fields as mutating in Swift. Unfortunately this doesn't work well with libstdc++, which uses mutable fields for some commonly used container types.

Our current user model assumes that we trust the `const` keyword on a C++ method, and import them as non-mutating in Swift. Let's make sure the tests reflect that.

Author: egorzhdan

test/Interop/Cxx/class/mutable-members-module-interface.swift

@@ -1,12 +1,10 @@
 // RUN: %target-swift-ide-test -print-module -module-to-print=MutableMembers -I %S/Inputs -source-filename=x -enable-experimental-cxx-interop | %FileCheck %s
 
-// REQUIRES: rdar91961524
-
 // CHECK: struct HasPublicMutableMember {
-// CHECK:   mutating func foo() -> Int32
+// CHECK:   func foo() -> Int32
 // CHECK:   var a: Int32
 // CHECK: }
 
 // CHECK: struct HasPrivateMutableMember {
-// CHECK:   mutating func bar()
+// CHECK:   func bar()
 // CHECK: }

test/Interop/Cxx/class/mutable-members-typechecker.swift

@@ -1,8 +1,7 @@
 // RUN: %target-typecheck-verify-swift -I %S/Inputs -enable-experimental-cxx-interop
-
-// REQUIRES: rdar91961524
+// expect-no-diagnostics
 
 import MutableMembers
 
-let obj = HasPublicMutableMember(a: 42) // expected-note {{change 'let' to 'var' to make it mutable}}
-let i = obj.foo() // expected-error {{cannot use mutating member on immutable value: 'obj' is a 'let' constant}}
+let obj = HasPublicMutableMember(a: 42)
+let i = obj.foo()