swift234

fix rdar://19179412, a regression introduced by my capture list rework patch.

This code is trying to avoid emitting multiple diagnostics on the same line:
before it was using a vector to keep track of exprs already emitted, I changed
it to clear the isImplicit() bit.  Clearing isImplicit introduces problems 
because various things (in this case, SourceRange validation) are keyed off 
whether an expression is implicit or not.

Instead of solving it either of those ways, fix our walk to just not recursively
descend into the 'self' argument of a self.foo expression when doing the walk.

While we're here, make it so that ChrisW's example doesn't even produce the 
diagnostic in question: the DRE is in a closure, but only because the entire
class is defined in the closure.  Stop the walker from descending recursively
into decls at all.


Swift SVN r23793