[Attributor] Introduce a helper do deal with constant type mismatches

If we simplify values we sometimes end up with type mismatches. If the
value is a constant we can often cast it though to still allow
propagation. The logic is now put into a helper and it replaces some
ad hoc things we did before.

This also introduces the AA namespace for abstract attribute related
functions and types.

Differential Revision: https://reviews.llvm.org/D103856