Convert ada-lang.c:map_matching_symbols

This converts ada-lang.c:map_matching_symbols to the callback
approach, merging the search loop and the call to
expand_symtabs_matching.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=16994
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=16998
Acked-By: Simon Marchi <simon.marchi@efficios.com>