Skip NonNull sema checks in unevaluated contexts.

Skip NonNull sema checks in unevaluated contexts.

Summary:
Currently when a function annotated with __attribute__((nonnull)) is called in an unevaluated context with a null argument a -Wnonnull warning is emitted.
This warning seems like a false positive unless the call expression is potentially evaluated. Change this behavior so that the non-null warnings use DiagRuntimeBehavior so they wont emit when they won't be evaluated.

Reviewers: majnemer, rsmith

Subscribers: mclow.lists, cfe-commits

Differential Revision: http://reviews.llvm.org/D13408

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@249787 91177308-0d34-0410-b5e6-96231b3b80d8