A scheme for more macro-matcher friendly pre-expansion gating

[Centril]

Pre-expansion gating will now avoid gating macro matchers that did not result in Success(...). That is, the following is now OK despite box 42 being a valid expr and that form being pre-expansion gated:

macro_rules! m {
    ($e:expr) => { 0 }; // This fails on the input below due to `, foo`.
    (box $e:expr, foo) => { 1 }; // Successful matcher, we should get `2`.
}

fn main() {
    assert_eq!(1, m!(box 42, foo));
}

Closes #65846.

r? @petrochenkov
cc @Mark-Simulacrum

[petrochenkov]

@bors r+

[bors]

📌 Commit bceaba8 has been approved by petrochenkov