Issue Description:

Clang has the Internals manual, and swiftc should have something similar. Particularly, I'm very interested in the following case being documented. Once it is, I feel like people would be better able to understand the compiler and add to it.

Let's say I have a type called SuperInt that has some cool integer operations that would be applicable and useful to everyone writing Swift. It's a given that this makes sense to be in the Swift language, and people are going to be using it so often that it makes sense to create a new literal syntax for this type. I'd like users to be able to instantiate a SuperInt with the following:

let superIntExample = %^100^%

Crazy syntax for a crazy example, but I wanted to make sure it's not something that's already in Swift. In this example, my SuperInt would be instantiated with a value of 100.

I'd like to know the parts of the Swift compiler I would need to understand and change to implement this new type with its new syntax.