Add “4.2” to ValidEffectiveVersions.

Author: tkremenek

include/swift/Basic/Version.h

@@ -143,8 +143,8 @@ class Version {
 
   // List of backward-compatibility versions that we permit passing as
   // -swift-version <vers>
-  static std::array<StringRef, 3> getValidEffectiveVersions() {
-    return {{"3", "4", "5"}};
+  static std::array<StringRef, 4> getValidEffectiveVersions() {
+    return {{"3", "4", "4.2", "5"}};
   };
 };