Macro `literal` fragment specifiers should forward to exact tokens

Motivating example (playground link):

macro_rules! foo {
    ($a:literal) => {
        foo!(@inner $a);
    };
    (@inner true) => {
        println!("true");
    };
    (@inner false) => {
        println!("false");
    };
}

fn main() {
    foo!(true);
    foo!(false);
}

The above fails to compile because literal does not forward to exact matches, but it doesn't seem like there is any reason it shouldn't. ident or tt can be used, but loosening restrictions can cause other problems.

This probably couldn't be changed without an edition depending on fallout, but we can probably make sure this works with macros 2.0.

The reference makes a comment about forwarding here: https://doc.rust-lang.org/nightly/reference/macros-by-example.html#forwarding-a-matched-fragment

Start with the Rust Reference section on forwarding a matched fragment and the linked macros 2.0 issue to understand the existing rules and proposed context. Determine the intended behavior for literal fragments matching exact true and false tokens, including any edition or compatibility requirements; done means the behavior is specified and implemented without unintended fallout.