is used for VarDecls that are immutable once defined. This
will eventually be used to model 'val' in SILGen, but for now
we can use it to optimize some 'self' situations.
At present, we use it for class 'self' in destructors and for
init methods of root classes. The init methods of derived
classes need to be able to mutate self when calling super.init
so they can't use this presently. I haven't gotten around to
switching general methods to use it yet.
This introduces two new regressions that don't appear in the
testsuite: we lose debug info for "self" in this case, and
we cannot close over self.
Swift SVN r10962
