[ModulePrinter] When printing decls in a module group, we print them according to their source order.

Source order preserves semantic information better than printing alphabetically.

lib/IDE/ModuleInterfacePrinting.cpp

@@ -308,6 +308,10 @@ void swift::ide::printSubmoduleInterface(
       continue;
     }
     if (FullModuleName.empty()) {
+      // If group name is given and the decl does not belong to the group, skip it.
+      if (GroupName && (!D->getGroupName() ||
+                        D->getGroupName().getValue() != GroupName.getValue()))
+        continue;
       // Add Swift decls if we are printing the top-level module.
       SwiftDecls.push_back(D);
     }
@@ -344,9 +348,15 @@ void swift::ide::printSubmoduleInterface(
   });
 
   std::sort(SwiftDecls.begin(), SwiftDecls.end(),
-            [](Decl *LHS, Decl *RHS) -> bool {
+            [&](Decl *LHS, Decl *RHS) -> bool {
     auto *LHSValue = dyn_cast<ValueDecl>(LHS);
     auto *RHSValue = dyn_cast<ValueDecl>(RHS);
+
+    // If group is specified, we order the decls by their source order.
+    if (GroupName && LHS->getSourceOrder() && RHS->getSourceOrder()) {
+      return LHS->getSourceOrder().getValue() < RHS->getSourceOrder().getValue();
+    }
+
     if (LHSValue && RHSValue) {
       StringRef LHSName = LHSValue->getName().str();
       StringRef RHSName = RHSValue->getName().str();
@@ -368,10 +378,6 @@ void swift::ide::printSubmoduleInterface(
 
   auto PrintDecl = [&](Decl *D) -> bool {
     ASTPrinter &Printer = *PrinterToUse;
-    if (GroupName && (!D->getGroupName() ||
-                      D->getGroupName().getValue() != GroupName.getValue()))
-      return false;
-
     if (!shouldPrint(D, AdjustedOptions)) {
       Printer.avoidPrintDeclPost(D);
       return false;