swift234

fix <rdar://problem/24530312> Swift ++fix-it produces bad code in nested expressions

The AvailabilityWalker was creating a new AvailabilityWalker instance whenever it
recursed through an assignexpr or memberrefexpr, which produced a new ExprStack.
This caused the fixit mechanics for migrating ++/-- to think that the ++/-- was
at the top level, when it wasn't.