[FunctionAttrs] Annotate "willreturn" for intrinsics
In D62801, new function attribute willreturn was introduced. In short, a function with willreturn is guaranteed to come back to the call site(more precise definition is in LangRef).
In this patch, willreturn is annotated for LLVM intrinsics.
Reviewed By: jdoerfert
Subscribers: jvesely, nhaehnle, sstefan1, llvm-commits
Differential Revision: https://reviews.llvm.org/D64904