Fix PR13910: Don't warn that __builtin_unreachable() is unreachable

Differential Revision: https://reviews.llvm.org/D25321

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@299951 91177308-0d34-0410-b5e6-96231b3b80d8
(cherry picked from commit eaaf906b8552a690efecd1a5cf5ebb8772e82504)
2 files changed