[cxx-interop] Clang member lookup should not look into Swift extensions

Calling `NominalTypeDecl::lookupDirect` triggers deserialization of Swift extensions for the type. `ClangRecordMemberLookup` shouldn't assume it is allowed to deserialize Swift extensions for the given C++ type: there might be extensions which reference the module that is currently being imported, which causes circular request dependency errors.

include/swift/AST/ClangModuleLoader.h

@@ -184,6 +184,11 @@ public:
   /// Imports a clang decl directly, rather than looking up its name.
   virtual Decl *importDeclDirectly(const clang::NamedDecl *decl) = 0;
 
+  /// Imports a clang decl from a base class, cloning it for \param newContext
+  /// if it wasn't cloned for this specific context before.
+  virtual ValueDecl *importBaseMemberDecl(ValueDecl *decl,
+                                          DeclContext *newContext) = 0;
+
   /// Emits diagnostics for any declarations named name
   /// whose direct declaration context is a TU.
   virtual void diagnoseTopLevelValue(const DeclName &name) = 0;