Add a test for an extremely confusing behavior of switches for
ExpressibleByNilLiteral-conforming types. From the looks of the expression
tree, one would hope that `case nil` would match such types. Instead, the
subject value is up-converted to an optional and compared to `nil` directly
with ~=.
This is an anti-pattern since the resulting value will never compare equal to `nil`, and the entire switch-case is dead. This appears to be a misfeature as the subject value is simply type-checked against an optional which produces an injection which matches the global ~= for Optionals.
Effectively, `case nil:` becomes `case $match ~= nil`.
rdar://89742267
