[flang] Move SAVE attribute checks to declaration checking

Constraint checking for explicit SAVE attributes is more
accurate when done along with other declaration checking, rather
than on the fly during name resolution.  This allows us to
catch attempts to attach explicit SAVE attributes to anything
that can't have one (constraints C859, C860).

Also delete `IsSave()`, whose few remaining uses were changed to the
more general `IsSaved()` predicate that seems more correct for
those uses, returning true for both explicit and implied SAVE
attributes.

Differential Revision: https://reviews.llvm.org/D146579