framework for varobj type mutation

framework for varobj type mutation

This patch introduces the framework necessary to support type mutations.
The only language that currently provides a language-specific hook for
that feature is Ada, but the hook remain unimplemented for now. The
actual implementation is tied to the rest of the varobj code for Ada,
and thus will be provided then.

gdb/ChangeLog:

        * varobj.c (ada_value_has_mutated): Add declaration.  New function.
        (struct language_specific): New field "value_has_mutated".
        (languages): Set field "value_has_mutated" in each entry of array.
        (varobj_value_has_mutated): New function.
        (varobj_udpdate): Add handling of type mutation.
        (value_of_root): Add handling of type mutation.
        (ada_value_has_mutated): New function.