[clang] Fix the canonicalization of paths in -fdiagnostics-absolute-paths

In the current implementation of clang the canonicalization of paths in
diagnostic messages (when using -fdiagnostics-absolute-paths) only works
if the symbolic link is in the directory part of the filename, not if
the file itself is a symbolic link to another file.

This patch adds support to canonicalize the complete path including the
file.

Reviewers: rsmith, hans, rnk, ikudrin

Reviewed By: rnk

Differential Revision: https://reviews.llvm.org/D70527