Fix segfault in hasDeclContext for nodes that have no decl context.

Summary:
Some declarations do not have a declaration context, like TranslationUnitDecl.
Fix hasDeclContext() to not segfault on these nodes.

Reviewers: klimek

Subscribers: klimek, cfe-commits

Differential Revision: http://reviews.llvm.org/D6003

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@220719 91177308-0d34-0410-b5e6-96231b3b80d8