[ModuleInterface] Fix silencing errors mode in swiftinterface rebuild

Make sure we disable forwarding diagnostics from the underlying instance
when building a swiftinterface in silencing errors mode.

include/swift/Frontend/ModuleInterfaceLoader.h

@@ -523,6 +523,7 @@ public:
                                            StringRef interfacePath,
                                            StringRef outputPath,
                                            SourceLoc diagLoc,
+                                           bool silenceErrors,
     llvm::function_ref<std::error_code(SubCompilerInstanceInfo&)> action) override;
 
   ~InterfaceSubContextDelegateImpl() = default;