Sema: Report public imports of private modules as errors by default

Align the behavior between release and debug compilers to always report
this as an error. Downstream compilers already make this an error.

rdar://136041870

Author: xymus

lib/Sema/TypeCheckDeclPrimary.cpp

@@ -2411,16 +2411,9 @@ class DeclChecker : public DeclVisitor<DeclChecker> {
         } else
           inFlight.fixItInsert(ID->getStartLoc(), "internal ");
 
-#ifndef NDEBUG
-        static bool enableTreatAsError = true;
-#else
-        static bool enableTreatAsError = getenv("ENABLE_PUBLIC_IMPORT_OF_PRIVATE_AS_ERROR");
-#endif
-
         bool isImportOfUnderlying = importer->getName() == target->getName();
         auto *SF = ID->getDeclContext()->getParentSourceFile();
-        bool treatAsError = enableTreatAsError &&
-                            !isImportOfUnderlying &&
+        bool treatAsError = !isImportOfUnderlying &&
                             SF->Kind != SourceFileKind::Interface;
         if (!treatAsError)
           inFlight.limitBehavior(DiagnosticBehavior::Warning);