[interop] namespace printing should only print redecls from one module interface

This ensures that when we're printing multiple module interfaces, we separate out
the namespaces and their members correctly between the produced module interfaces.

Author: hyp

lib/AST/ASTPrinter.cpp

@@ -2300,7 +2300,15 @@ static void addNamespaceMembers(Decl *decl,
 
   // This is only to keep track of the members we've already seen.
   llvm::SmallPtrSet<Decl *, 16> addedMembers;
+  const auto *declOwner = namespaceDecl->getOwningModule();
+  if (declOwner)
+    declOwner = declOwner->getTopLevelModule();
   for (auto redecl : namespaceDecl->redecls()) {
+    // Skip namespace declarations that come from other top-level modules.
+    if (const auto *redeclOwner = redecl->getOwningModule()) {
+      if (declOwner && declOwner != redeclOwner->getTopLevelModule())
+        continue;
+    }
     for (auto member : redecl->decls()) {
       if (auto classTemplate = dyn_cast<clang::ClassTemplateDecl>(member)) {
         // Add all specializations to a worklist so we don't accidently mutate

test/Interop/Cxx/namespace/cross-module-redecls-module-interface.swift

@@ -0,0 +1,49 @@
+// RUN: rm -rf %t
+// RUN: split-file %s %t
+// RUN: %target-swift-ide-test -print-module -module-to-print=ModuleB -I %t/Inputs -source-filename=x -enable-experimental-cxx-interop | %FileCheck --check-prefix=CHECKB %s
+// RUN: %target-swift-ide-test -print-module -module-to-print=ModuleA -I %t/Inputs -source-filename=x -enable-experimental-cxx-interop | %FileCheck --check-prefix=CHECKA %s
+
+//--- Inputs/module.modulemap
+module ModuleA {
+    header "headerA.h"
+    requires cplusplus
+}
+
+module ModuleB {
+    header "headerB.h"
+    requires cplusplus
+}
+
+//--- Inputs/headerA.h
+
+namespace ns {
+    struct A {
+        int x;
+    };
+}
+
+//--- Inputs/headerB.h
+
+#include "headerA.h"
+
+namespace ns {
+    struct B {
+        int y;
+    };
+}
+
+// CHECKB:     enum ns {
+// CHECKB-NEXT: struct B {
+// CHECKB-NEXT:    init()
+// CHECKB-NEXT:    init(y: Int32)
+// CHECKB-NEXT:    var y: Int32
+// CHECKB-NEXT:  }
+// CHECKB-NEXT: }
+
+// CHECKA:     enum ns {
+// CHECKA-NEXT: struct A {
+// CHECKA-NEXT:    init()
+// CHECKA-NEXT:    init(x: Int32)
+// CHECKA-NEXT:    var x: Int32
+// CHECKA-NEXT:  }
+// CHECKA-NEXT: }