Diagnose when trying to import non-ossa module to ossa when implicit modules are disabled

include/swift/AST/DiagnosticsSema.def

@@ -901,6 +901,9 @@ ERROR(serialization_target_too_new_repl,none,
       "deployment target of %0 %3: %4",
       (StringRef, llvm::VersionTuple, Identifier, llvm::VersionTuple,
        StringRef))
+ERROR(serialization_non_ossa_module_incompatible, Fatal,
+      "cannot import non-OSSA module into an OSSA module",
+      (Identifier))
 
 ERROR(serialization_fatal,Fatal,
       "fatal error encountered while reading from module '%0'; "

lib/Serialization/SerializedModuleLoader.cpp

@@ -1190,7 +1190,12 @@ void swift::serialization::diagnoseSerializedASTLoadFailure(
                        moduleBufferID);
     break;
   case serialization::Status::NotInOSSA:
-    // soft reject, silently ignore.
+    // Diagnose only when explicit build modules is enabled
+    if (Ctx.SerializationOpts.ExplicitModuleBuild) {
+      Ctx.Diags.diagnose(diagLoc,
+                         diag::serialization_non_ossa_module_incompatible,
+                         ModuleName);
+    }
     break;
   case serialization::Status::RevisionIncompatible:
     Ctx.Diags.diagnose(diagLoc, diag::serialization_module_incompatible_revision,