swift234

Jordan pointed out that VarDecl::isSelfParameter really does need
to check the implicit bit for decls, because otherwise we'd consider
params declared with a name of `self` as being "the self parameter".

This is trivial, except for the fact that we don't serialize the
implicit bit on parameters.  I can't bring myself to burn encoding
space for this (particularly since we shouldn't be encoding self
decls in the first place!), so make the deserializer infer this bit
instead.