[DNM] Enable deserialization safety by default for testing

Author: xymus

include/swift/Basic/LangOptions.h

@@ -387,8 +387,7 @@ namespace swift {
 
     /// Enable early skipping deserialization of decls that are marked as
     /// unsafe to read.
-    bool EnableDeserializationSafety =
-      ::getenv("SWIFT_ENABLE_DESERIALIZATION_SAFETY");
+    bool EnableDeserializationSafety = true;
 
     /// Whether to enable the new operator decl and precedencegroup lookup
     /// behavior. This is a staging flag, and will be removed in the future.