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

llvm-svn: 220719