[ir] Let void struct type represent void type

[lin-hitonami]

This PR lets kernel->ret_type and kernel->args_type be a void struct type instead of nullptr when there is no return value or argument.

Stack from ghstack (oldest at bottom):

• [refactor] Migrate {set, get}_{arg, ret} functions from RuntimeContext to LaunchContextBuilder #7550
• [llvm] Compile the kernel arguments to a StructType #7441
• [bug] Fix crashing on printing FrontendFuncCallStmt with no return value #7561
• [refactor] Remove LaunchContextBuilder::set_arg_raw #7560
• [llvm] Generalize TaskCodeGenLLVM::create_return to set_struct_to_buffer #7558
• -> [ir] Let void struct type represent void type #7548
• [aot] Let C-API use LaunchContextBuilder to manage RuntimeContext #7547
• [ir] Let the reference type declare a pointer argument #7546

[ailzhang]

LGTM!

[lin-hitonami]

/land