[Migrator] Set language version to 4 for the fix-it passes

When running the fix-it passes, we should explicitly set the effective
language version to 4, since passing language version 3 may be the
default at first.

rdar://problem/31854159

Author: bitjammer

lib/Migrator/Migrator.cpp

@@ -96,6 +96,7 @@ Migrator::performAFixItMigration() {
   CompilerInvocation Invocation { StartInvocation };
   Invocation.clearInputs();
   Invocation.addInputBuffer(InputBuffer.get());
+  Invocation.getLangOptions().EffectiveLanguageVersion = { 4, 0, 0 };
 
   CompilerInstance Instance;
   if (Instance.setup(Invocation)) {