Make ada_lookup_encoded_symbol "return" a struct ada_symbol_info

Make ada_lookup_encoded_symbol "return" a struct ada_symbol_info

This makes ada_lookup_encoded_symbol more consistent with other functions
such as ada_lookup_symbol_list, and also makes it clearer in the code
using that function that symbol and block are related.

gdb/ChangeLog:

        * ada-lang.c (ada_lookup_encoded_symbol): Now returns void.
        Replace block_found argument by symbol_info.  Adjust
        implementation accordingly.  Add function documentation.
        (ada_lookup_symbol): Adjust to new ada_lookup_encoded_symbol.
        Fix documentation.
        * ada-lang.h (ada_lookup_encoded_symbol): Update declaration.
        * ada-exp.y (write_object_renaming): Adjust to new
        ada_lookup_encoded_symbol API.