[FunctionAttrs] Derive willreturn for fns with readonly` & `mustprogress`.

Similar to D94125, derive `willreturn` for functions that are `readonly` and
`mustprogress` in FunctionAttrs.

To quote the reasoning from D94125:

    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: jdoerfert, nikic

Differential Revision: https://reviews.llvm.org/D94502