[analyzer][ctu] fix unsortable diagnostics

Summary: In the provided test case the PathDiagnostic compare function was not able to find a difference.

Reviewers: xazax.hun, NoQ, dcoughlin, george.karpenkov

Reviewed By: george.karpenkov

Subscribers: a_sidorin, szepet, rnkovacs, a.sidorin, mikhail.ramalho, cfe-commits

Differential Revision: https://reviews.llvm.org/D48474

llvm-svn: 336275