[AA] Rename FunctionModRefBehavior to MemoryEffects (NFC)

As part of https://discourse.llvm.org/t/rfc-unify-memory-effect-attributes/65579,
the FunctionModRefBehavior class sees a good bit of additional use,
and I've found the name to be something of a mouthful. This patch
renames it to MemoryEffects, which has a couple of advantages over
the old name:
 * It is more concise.
 * It decouples it from modelling only functions.
 * It matches the terminology of the aforementioned RFC.
 * The meaning should be more obvious to people not familiar with
   our particular AA lingo.

This patch just updates the class definition. Other uses of the
name will be updated separately.

Differential Revision: https://reviews.llvm.org/D135962