Automate attribute argument count semantic checking when there are variadic or optional arguments present. With this, the only time you should have to manually check attribute argument counts is when HasCustomParsing is set to true, or when you have variadic arguments that aren't really variadic (like ownership_holds and friends).

Updating the diagnostics in the launch_bounds test since they have been improved in that case. Adding a test for nonnull since it has little test coverage, but has truly variadic arguments.