swift234

Simplify the implementation of SILGenFunction::VarLoc by eliminating the
"isConstant" distinction.  This was an irritating bit of redundant state
that was making the code more complicated.  Clients of VarLoc really only
care about "has address" and "has box", not whether the VarLoc came from
a let or var (and if they did, they can ask the VarDecl directly).  NFC,
just more "yak shaving" as Doug likes to say.


Swift SVN r23869