Annotate return values of allocation functions with dereferenceable_or_null

Summary:
Example
define dso_local noalias i8* @_Z6maixxnv() local_unnamed_addr #0 {
entry:
  %call = tail call noalias dereferenceable_or_null(64) i8* @malloc(i64 64) #6
  ret i8* %call
}


Reviewers: jdoerfert

Reviewed By: jdoerfert

Subscribers: aaron.ballman, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D66651

llvm-svn: 370168