Convert filename-seen-cache.h to new hash table

This converts filename-seen-cache.h to use the new hash table.
filename-seen-cache.c is removed.

Change-Id: Iffac1d5e49d1610049b7deeef6e98d49e644366a
Co-Authored-By: Tom Tromey <tom@tromey.com>
Approved-By: Tom Tromey <tom@tromey.com>
3 files changed