[analyzer] Fix filename in cross-file HTML report

Summary:
The filename is currently taken from the start of the path, while the
line and column are taken from the end of the path.
This didn't matter until cross-file path reporting was added.

Reviewers: george.karpenkov, dcoughlin, vlad.tsyrklevich

Reviewed By: george.karpenkov, vlad.tsyrklevich

Subscribers: xazax.hun, szepet, a.sidorin, cfe-commits

Differential Revision: https://reviews.llvm.org/D45611

llvm-svn: 331361