[Attributor] Deduce "willreturn" function attribute

Summary:
Deduce the "willreturn" attribute for functions.

For now, intrinsics are not willreturn. More annotation will be done in another patch.

Reviewers: jdoerfert

Subscribers: jvesely, nhaehnle, nicholas, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D63046

llvm-svn: 366335