There is a small bug fix here in the identification of the catch node,
where the leading `{` of a closure was considered to be "inside" the
closure for code like
{ ... }()
causing us to assume that the call to the closure would catch the error
within the closure.
Other than that, introduce the thrown error type into the type checker's
modeling of `withoutActuallyEscaping(_:do:)`, and mirror that in the
library declaration.
withoutActuallyEscaping has a signature like `<T..., U, V, W> (@nonescaping (T...) throws<U> -> V, (@escaping (T...) throws<U> -> V) -> W) -> W, but our type system for functions unfortunately isn't quite that expressive yet, so we need to special-case it. Set up the necessary type system when resolving an overload set to reference withoutActuallyEscaping, and if a type check succeeds, build a MakeTemporarilyEscapableExpr to represent it in the type-checked AST.
