[FunctionAttrs] Annotate "willreturn" for intrinsics

[FunctionAttrs] Annotate "willreturn" for intrinsics

Summary:
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.

Reviewers: jdoerfert

Reviewed By: jdoerfert

Subscribers: jvesely, nhaehnle, sstefan1, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D64904

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@367184 91177308-0d34-0410-b5e6-96231b3b80d8