[TableGen] Use MemoryEffects to represent intrinsic memory effects (NFCI)

The TableGen implementation was using a homegrown implementation of
FunctionModRefInfo. This switches it to use MemoryEffects instead.
This makes the code simpler, and will allow exposing the full
representational power of MemoryEffects in the future. Among other
things, this will allow us to map IntrHasSideEffects to an
inaccessiblemem readwrite, rather than just ignoring it entirely
in most cases.

To avoid layering issues, this moves the ModRef.h header from IR
to Support, so that it can be included in the TableGen layer.

Differential Revision: https://reviews.llvm.org/D137641