[Sema] Force reporting public imports of private module as error with asserts

When running in assert mode make it an error for a public module to
publicly import a private module.

Author: xymus

lib/Sema/TypeCheckDeclPrimary.cpp

@@ -1776,6 +1776,9 @@ class DeclChecker : public DeclVisitor<DeclChecker> {
         }
 
         static bool treatAsError = getenv("ENABLE_PUBLIC_IMPORT_OF_PRIVATE_AS_ERROR");
+#ifndef NDEBUG
+        treatAsError = true;
+#endif
         if (!treatAsError)
           inFlight.limitBehavior(DiagnosticBehavior::Warning);
       }

test/Sema/implementation-only-import-suggestion.swift

@@ -34,11 +34,11 @@
 // RUN:   -library-level other -D PUBLIC_IMPORTS
 #if PUBLIC_IMPORTS
 import PublicSwift
-import PrivateSwift // expected-warning{{private module 'PrivateSwift' is imported publicly from the public module 'main'}}
+import PrivateSwift // expected-error{{private module 'PrivateSwift' is imported publicly from the public module 'main'}}
 
 import PublicClang
-import PublicClang_Private // expected-warning{{private module 'PublicClang_Private' is imported publicly from the public module 'main'}}
-import FullyPrivateClang // expected-warning{{private module 'FullyPrivateClang' is imported publicly from the public module 'main'}}
+import PublicClang_Private // expected-error{{private module 'PublicClang_Private' is imported publicly from the public module 'main'}}
+import FullyPrivateClang // expected-error{{private module 'FullyPrivateClang' is imported publicly from the public module 'main'}}
 import main // expected-warning{{'implementation-only-import-suggestion.swift' is part of module 'main'; ignoring import}}
 
 /// Expect no warnings with implementation-only imports.