Use symbol search name in expand_symtabs_matching_via_partial...

Use symbol search name in expand_symtabs_matching_via_partial...

We are iterating over all symbols in a partial symtab that would
match a given name, so we should match the partial symbols
search name against the given name rather than using the natural
name.  In C++, that does not make a difference, but it does in
Ada, because Ada searches using the symbol encoded name...

We also update the generation of the .gdb_index file to match this
change in the search. Although technically an incompatible change,
we do not increment the gdb_index version number, because Ada is
the only language where it would make a difference - except that
this feature is not supported for Ada.

gdb/ChangeLog:

        * psymtab.c (expand_symtabs_matching_via_partial): Match
        the partial symbols using their SYMBOL_SEARCH_NAME.
        * symfile.h (struct quick_symbol_functions): Udate the
        documentation of expand_symtabs_matching.
        * dwarf2read.c (write_psymbols): Use SYMBOL_SEARCH_NAME instead
        of SYMBOL_NATURAL_NAME in index entry.