ada_evaluate_subexp<OP_VAR_VALUE>: Avoid static fixing when possible.

ada_evaluate_subexp<OP_VAR_VALUE>: Avoid static fixing when possible.

Now that the OP_VAR_VALUE section of this function has been reorganized
a bit, we can fall-back on standard evaluation when static fixing is
not required. This patch does that, but being exclusive about when
static fixing has to be used, rather than doing it all the time when
noside is EVAL_AVOID_SIDE_EFFECTS.

This will pave the way for later when we want to evaluate entities
that have no GNAT encodings related to them but dynamic properties
instead. In that case, we expect the standard evaluation to resolve
those dynamic properties for us, even in no-side-effect mode.

gdb/ChangeLog:

        * ada-lang.c (ada_evaluate_subexp) <OP_VAR_VALUE>:
        When noside is EVAL_AVOID_SIDE_EFFECTS, only return a statically
        fixed value for records and unions for which some GNAT encodings
        are present.