Varobj support for Ada.

Varobj support for Ada.

This patch adds varobj support for Ada variables.  Most of the code
is implemented in a separate Ada-specific file called ada-varobj.c.
The only bits in varobj.c are the functions used as the hooks in
the language-specific varobj's vector.

gdb/ChangeLog:

        * ada-varobj.h, ada-varobj.c: New files.
        * Makefile.in (SFILES): Add ada-varobj.c.
        (HFILES_NO_SRCDIR): Add ada-varobj.h.
        (COMMON_OBS): Add ada-varobj.o.