There was a special case here to type-check `T.init` as a single closure
`{ args.. in T.init(args..) }`, but really, we can do that for any static
member applied to a static metatype base, including operators.
Also fix SILGen's function conversion peephole so it looks through
`as (T...) -> U` coercions that don't involve bridging.
Remove the preallocated closure discriminator from KeyPathExpr and go back
to expanding them using an AutoClosureExpr inside of a CaptureListExpr now
that that's supported. This allows the discriminator to be assigned during
type checking without disturbing the indexing of explicit closure literals.
Sometimes we emit a closure literal with escaping/nonthrowing/nonasync type
into a context that wants a nonescaping/throwing/async function, and it
ends up wrapped in a conversion. We can look through any of these and emit
the closure literal directly with those effects.
