[TBAA] Model call accessing immutable type as readnone

Accesses to constant memory are not observable and should be
reported as readnone, not readonly. This is consistent with what
we do for normal (non-call) instructions: For those, the TBAA
metadata will result in pointsToConstantMemory() returning true,
which will then result in a NoModRef result, not a Ref result.

Differential Revision: https://reviews.llvm.org/D135864