[Sema] Fix crash in getConstructorName.

Summary:
Can happen when getConstructorName is called on invalid decls,
specifically the ones that do not have the injected class name.

Reviewers: bkramer, rsmith

Reviewed By: rsmith

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D48880

llvm-svn: 336244