* freeze.adb (Freeze_Expression): Allow freezing of static
scalar subtypes that are prefixes of an attribute, even if not
yet marked static. Such attributes will get marked as static
later in Eval_Attribute (as called from Resolve_Attribute).
* sem_attr.adb (Eval_Attribute): Remove wrong code that does an
early return for attribute prefixes that are unfrozen source-level
types. This code was incorrectly bypassing folding of unfrozen
static subtype attributes in default expressions (the executable
example in the now-deleted comment was in fact illegal).