gdb: change objfile::map_symtabs_matching_filename to find_symtab_matching_filename

The only user of objfile::map_symtabs_matching_filename uses that method
to find the first matching symtab.  It would therefore be more natural
for that method to be a "find" method, returning the first symtab
matching the predicate.

Change map_symtabs_matching_filename to be
find_symtab_matching_filename, and the internal
iterate_over_one_compunit_symtab to be find_symtab_in_compunit_symtab.

This makes function find_symtab simpler.

Change-Id: Id14a95498fad243495d6eab18810d0c4ab8dbf90
Approved-By: Andrew Burgess <aburgess@redhat.com>