Convert compile-c-symbols.c to new hash table

Convert compile-c-symbols.c to new hash table

This converts compile-c-symbols.c to use the new hash table.

I made it use a set of string_view instead of a set of `symbol *`, to
avoid calling `symbol::natural_name` over and over.  This appears safe
to do, since I don't expect the storage behing the natural names to
change during the lifetime of the map.

Change-Id: Ie9f9334d4f03b9a8ae6886287f82cd435eee217c
Co-Authored-By: Tom Tromey <tom@tromey.com>
Approved-By: Tom Tromey <tom@tromey.com>