[nfc] Add 'is' in diagnostic message.

Author: zoecarver

include/swift/AST/DiagnosticsClangImporter.def

@@ -203,7 +203,7 @@ NOTE(get_last_element, none, "Do you want to get the last element instead?",
      ())
 
 NOTE(iterator_method_unavailable, none, "C++ method '%0' that returns an "
-                                  "iterator unavailable",
+                                  "iterator is unavailable",
      (StringRef))
 NOTE(iterator_potentially_unsafe, none, "C++ methods that return iterators "
                                         "are potentially unsafe. Try re-writing "

test/Interop/Cxx/class/invalid-unsafe-projection-errors.swift

@@ -37,7 +37,7 @@ public func test(x: M) {
   // CHECK: note: Mark method 'test3' as 'safe_to_import' in C++ to make it available in Swift.
   // CHECK: note: Mark type 'Ptr' as 'self_contained' in C++ to make methods that use it available in Swift.
   x.test3()
-  // CHECK: note: C++ method 'begin' that returns an iterator unavailable
+  // CHECK: note: C++ method 'begin' that returns an iterator is unavailable
   // CHECK: note: C++ methods that return iterators are potentially unsafe. Try re-writing to use Swift iterator APIs.
   x.begin()
 }