swift234

Fix <rdar://22774938> QoI: "never used" in an "if let" should rewrite expression to use != nil

When we see an unused variable in a simple-enough "if/let" (also guard and
while of course), fixit it into a comparison against nil instead of replacing
the name of the variable with "_".  Also special case initialization with an
as? expression, since we can transform that into an "is" boolean test.