Diagnostic for requiring [raypayload] attribute could be clearer

Description
When using a payload without the [raypayload] attribute on the struct, you get a diagnostic error pointing at the variable declaration, such as:

<file:line>: error: type 'Payload' used as payload requires that it is annotated with the [raypayload] attribute
MyPayload pld;
          ^

The problem is with the type used, so just pointing at the variable declaration does not show you where the attribute was expected. It should add a note pointing to the struct declaration, and provide a fixit for the location of the attribute, since the location is not very intuitive: between the struct and the name of the struct.

Steps to Reproduce
https://godbolt.org/z/7sq5q17h6

Start with the Godbolt reproduction linked in the issue and inspect the diagnostic emitted when a payload type lacks the [raypayload] attribute. The work is done when the diagnostic points to the struct declaration, includes a note showing where the attribute is expected, and offers a fix-it for the location between struct and the struct name.