[cxx-interop] Fix namespace lookup

If a namespace has redecls in different modules, Swift sometimes picked a redecl which is not visible from the requested module, and wrongly removed the namespace from lookup results.

This resulted in errors like this:
```
*** DESERIALIZATION FAILURE ***
Could not deserialize type for 'init(cxxString:)'
Caused by: top-level value not found (std)
Cross-reference to module '__ObjC'
... std
```

Author: egorzhdan

lib/ClangImporter/ClangImporter.cpp

@@ -2725,7 +2725,7 @@ static bool isVisibleFromModule(const ClangModuleUnit *ModuleFilter,
   // Handle redeclarable Clang decls by checking each redeclaration.
   bool IsTagDecl = isa<clang::TagDecl>(D);
   if (!(IsTagDecl || isa<clang::FunctionDecl>(D) || isa<clang::VarDecl>(D) ||
-        isa<clang::TypedefNameDecl>(D))) {
+        isa<clang::TypedefNameDecl>(D) || isa<clang::NamespaceDecl>(D))) {
     return false;
   }