[Lint] Permit aliasing noalias readonly arguments

Summary:
If two arguments are both readonly, then they have no memory dependency
that would violate noalias, even if they do actually overlap.

Reviewers: hfinkel, efriedma

Reviewed By: efriedma

Subscribers: efriedma, hiraditya, llvm-commits, tstellar

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D60239

llvm-svn: 359047