Handle other cases than EVAL_NORMAL in the default case

Handle other cases than EVAL_NORMAL in the default case

In the evaluation of an expression in Ada mode, the default case
unwraps the argument unconditionally. For an object of a variant
record type, this unwrapping builds a fixed type from the
specification of the variant type and the actual values of the
object's discriminants.  It means that unwrapping needs the "proper"
value for the object, not just a zero value with the proper type.

When not in EVAL_NORMAL, we cannot assume that the evaluation returns
such a proper value; it may well return a zero value of the
appropriate type e.g in EVAL_AVOID_SIDE_EFFECTS. It is wrong to try to
unwrap in that case.

In particular, a problem shows up when using expression of the form
{VARIANT_TYPE}OBJ. GDB first evaluates this expression in
EVAL_AVOID_SIDE_EFFECTS to compute the type, the evaluation of OBJ
in most cases returns a zero value of its type, and as UNOP_MEMVAL
is mapped to the default case its evaluation ends up trying to
read memory around address 0.

gdb/ChangeLog:

	* ada-lang.c (ada_evaluate_subexp): Unwrap only in EVAL_NORMAL.