[analyzer] Diagnostics: Ensure that the default end of diagnostic path

piece can always be generated.

The default end of diagnostic path piece was failing to generate on a
BlockEdge that was outgoing from a basic block without a terminator,
resulting in a very simple diagnostic being rendered (ex: no path
highlighting or custom visitors). Reuse another function, which is
essentially doing the same thing and correct it not to fail when a block
has no terminator.

llvm-svn: 150659