Remove all limitations on variables in result builders

Before Swift 5.8, declaring local variables in result builders had many constraints.

Local variables need to:

• Have an initializer expression
• Cannot be computed
• Cannot have observers
• Cannot have attached property wrappers
• And other constraints

These constraints contradict the result builder proposal, which states that local declarations are unaffected by the transformation.

Declaration statements
Local declarations are left alone by the transformation. This allows developers to factor out subexpressions freely to clarify their code, without affecting the result builder transformation.

Before Swift 5.8, you couldn't declare variables in any of these forms.

struct FooViewBuilder: View {
    var body: some View {
        var computedVar: String {
            return "Computed"
        }
        lazy var lazyVar = compute()
        let uninitializedVar: String
        let defaultNilVar: String?
        @AppStorage("example_string") var propertyWrapperVar = "Foo"

        VStack {
            Text("Foo")
        }
    }

    func compute() -> String {
        return "Foo"
    }
}

Swift 5.8 (SE-0373) removes all of these limitations. So, the above code would build without any errors in Swift 5.8.