[Ada] New functions to decode Ada types and values

[Ada] New functions to decode Ada types and values

This patch introduces two new functions that will be used to support
the implementation of the ada-varobj effort.  The function descriptions
should say it all...

gdb/ChangeLog:

        * ada-lang.h (ada_get_decoded_value, ada_get_decoded_type): Add
        declaration.
        * ada-lang.c (ada_get_decoded_value, ada_get_decoded_type): New
        function.