[Attributor] Deduce memory behavior of functions and arguments

Deduce the memory behavior, aka "read-none", "read-only", or
"write-only", for functions and arguments.

Reviewers: sstefan1, uenoku

Subscribers: hiraditya, bollu, jfb, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D67384

llvm-svn: 373965