Set the default compiler effective language version to Swift 5 for now

Author: shahmishal

include/swift/Basic/LangOptions.h

@@ -172,7 +172,7 @@ namespace swift {
     ///
 
     /// User-overridable language version to compile for.
-    version::Version EffectiveLanguageVersion = version::Version::getCurrentLanguageVersion();
+    version::Version EffectiveLanguageVersion = Version{5};
 
     /// Swift runtime version to compile for.
     version::Version RuntimeVersion = version::Version::getCurrentLanguageVersion();