[Attributor] Use the AANoNull attribute directly in AADereferenceable

Summary:
Instead of constantly keeping track of the nonnull status with the
dereferenceable information we can simply query the nonnull attribute
whenever we need the information (debug + manifest).

Reviewers: sstefan1, uenoku

Subscribers: hiraditya, bollu, jfb, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D66113

llvm-svn: 368924