TypeCheckType: Do not emit migration mode diags if `nonisolated(nonsending)` is explicit
This broke when we split `@execution(...)` into `@concurrent` and
`nonisolated(nonsending)` because the latter became its own `TypeRepr`,
whereas the condition for whether to attempt migration diagnostics
inside `resolveASTFunctionType` is still based on the function type's
attributes alone.
(cherry picked from commit bee2b6778e207edb0c256d9fb717f08824f7e70a)