Add assert for 'bad' code path in GetUniqueNamespaceDeclaration

Summary:
If we call this function with a non-namespace as a second argument (and a nullptr name), we currently
only get a nullptr as a return when we hit the "Bad!!!" code path. This patch just adds an assert as this
seems to be a programming error in the calling code.

Reviewers: shafik

Subscribers: abidh, lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D57880

llvm-svn: 365157