[spec] Apparent type of string literal type should be global interface String

[mhegazy]

Section 3.8.1 should be augmented to say that a string literal type should have as its apparent type the interface String.

This is to enable things like:

declare function f(x: "str");
declare function f(x: String);

The new compiler is already doing this. We just need to change the spec and add a test for it.