[interop] fix swift-ide-test module interface namespace printing crash for _impl namespace in swift_private namespace

Author: hyp

lib/ClangImporter/ImportDecl.cpp

@@ -1106,6 +1106,9 @@ namespace {
         auto parentNS = cast<clang::NamespaceDecl>(decl->getParent());
         auto parent =
             Impl.importDecl(parentNS, getVersion(), /*UseCanonicalDecl*/ false);
+        // The parent namespace might not be imported if it's `swift_private`.
+        if (!parent)
+            return nullptr;
         dc = cast<EnumDecl>(parent);
       }
 

test/Interop/SwiftToCxxToSwift/hide-swift-module-namespace-in-swift.swift

@@ -9,6 +9,8 @@
 
 // RUN: %check-interop-cxx-header-in-clang(%t/swiftMod2.h -DSWIFT_CXX_INTEROP_HIDE_STL_OVERLAY  -Wno-error)
 
+// RUN: %target-swift-ide-test -print-module -module-to-print=SwiftToCxxTest -I %t -source-filename=x -enable-experimental-cxx-interop -Xcc -DSWIFT_CXX_INTEROP_HIDE_SWIFT_ERROR
+
 // XFAIL: OS=linux-android, OS=linux-androideabi
 
 //--- header.h