[Migrator] Don't run AST passes when in Swift 4 or later

AST passes assume that you are migrating from a version earlier
than Swift 4, where declaration references and type names may be
unconditionally renamed if their USRs match.

For example, this can happen for TypeMemberDiffItem entries where the
Objective-C USR is the same in Swift 3 and Swift 4, but the type is
spelled differently in Swift 4. A concrete example of this is:

`NSDocumentTypeDocumentAttribute` (Swift 3) ->
  `NSAttributedString.DocumentAttributeKey` (Swift 4).

Although this declaration is imported differently in Swift 4, its
Objective-C USR is `c:@NSDocumentTypeDocumentAttribute` for both.

rdar://problem/32604558

lib/Migrator/Migrator.cpp

@@ -53,9 +53,11 @@ bool migrator::updateCodeAndEmitRemap(CompilerInstance *Instance,
   }
 
   // Phase 2: Syntactic Transformations
-  auto FailedSyntacticPasses = M.performSyntacticPasses();
-  if (FailedSyntacticPasses) {
-    return true;
+  if (Invocation.getLangOptions().EffectiveLanguageVersion[0] < 4) {
+    auto FailedSyntacticPasses = M.performSyntacticPasses();
+    if (FailedSyntacticPasses) {
+      return true;
+    }
   }
 
   // Phase 3: Post Fix-it Passes