[Attributor] Derive `willreturn` based on `mustprogress`

Since D86233 we have `mustprogress` which, in combination with
`readonly`, implies `willreturn`. The idea is that every side-effect
has to be modeled as a "write". Consequently, `readonly` means there
is no side-effect, and `mustprogress` guarantees that we cannot "loop"
forever without side-effect.

Reviewed By: fhahn

Differential Revision: https://reviews.llvm.org/D94125