Fix crash for incomplete labels in macros.

Still the formatting can be improved, but at least we don't assert any
more. This happened when trying to format lib/Sema/SemaType.cpp.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@175003 91177308-0d34-0410-b5e6-96231b3b80d8