[include-cleaner] Loose matching for verbatim headers

A verbatim header usually corresponds to a symbol from a header with
a pragma "IWYU pragma: private, include <foo.h>".

Currently this is only satisfied if the main file contains exactly
  #include <foo.h>
In practice this is too strict, we also want to allow
  #include "path/to/foo.h"
so long as they resolve to the same file.

We cannot be 100% sure without doing IO, and we're not willing to do
that, but we can detect the common cases based on paths.

Differential Revision: https://reviews.llvm.org/D155819