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