Improve performance of Ada name searches

A user reported that certain operations -- like printing a large
structure -- could be slow.  I tracked this down to
ada-lang.c:map_matching_symbols taking an inordinate amount of time.
Specifically, calls like the one to look for a parallel "__XVZ"
variable, in ada_to_fixed_type_1, could result in gdb walking over all
the entries in the cooked index over and over.

Looking into this reveals that
cooked_index_functions::expand_matching_symbols is not written
efficiently -- it ignores its "ordered_compare" parameter.  While
fixing this would be good, it turns out that this entire method isn't
needed; so this series removes it.

However, the deletion is not done in this patch.  This one, instead,
fixes the immediate cause of the slowdown, by using
objfile::expand_symtabs_matching when possible.  This approach is
faster because it is more selective about which index entries to
examine.