[clang] prevent crash for nonnull attribut in constant context (Bug 43601)

Summary:

bug : https://bugs.llvm.org/show_bug.cgi?id=43601

Reviewers: rnk

Reviewed By: rnk

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D68716

llvm-svn: 374285