[AccessorMacro] disallow accessor macro apply to variable declaration

[RayZhao1998]

Resolve #2491

[ahoppen]

This change will only disallow accessor macros on let variables in assertMacroExpansion, not during compilation. Disallowing it during compilation needs to be done in the compiler repository. I would prefer not to have a difference in behavior, so those changes would need to be done in sync.

[RayZhao1998]

All right. Going to close this PR for now. Maybe I can try to make it done in the compiler repository 😀.

[ahoppen]

If you are interested in making the change in the compile repo, it’s probably a var->isLet check that needs to be added for accessors that aren’t valid for let declarations somewhere around here

https://github.com/apple/swift/blob/f7c9966420051fb09926704c2f6d9001242c7e64/lib/Sema/TypeCheckMacros.cpp#L1625