Don't create a compile time binding after CompileTimeBindingInVarDecl (#4463)

Related to #4461, more generally try not to produce a compile-time
binding when the code shouldn't be able to do so.

toolchain/check/handle_binding_pattern.cpp

@@ -93,6 +93,8 @@ static auto HandleAnyBindingPattern(Context& context, Parse::NodeId node_id,
             CompileTimeBindingInVarDecl, Error,
             "`var` declaration cannot declare a compile-time binding");
         context.emitter().Emit(type_node, CompileTimeBindingInVarDecl);
+        // Prevent lambda helpers from creating a compile time binding.
+        needs_compile_time_binding = false;
       }
       auto binding_id =
           is_generic