[Attributor] NoAlias on return values.

Porting function return value attribute noalias to attributor.
This will be followed with a patch for callsite and function argumets.

Reviewers: jdoerfert

Subscribers: lebedev.ri, hiraditya, llvm-commits

Differential Revision: https://reviews.llvm.org/D63067

llvm-svn: 366728