rustdoc doctest: detect fn main after an unexpected semicolon

[notriddle]

Fixes #91014

The basic problem with this is that rustdoc, when hunting for fn main, will stop parsing after it reaches a fatal error. This unexpected semicolon was a fatal error, so in src/test/rustdoc-ui/failed-doctest-extra-semicolon-on-item.rs, it would wrap the doctest in an implied main function, turning it into this:

fn main() {
    struct S {};
    fn main() {
        assert_eq!(0, 1);
    }
}

This, as it turns out, is totally valid, and it executes no assertions, so it passes, even though the user wanted it to execute the assertion.

The Rust parser already has the ability to recover from these unexpected semicolons, but to do so, it needs to use the parse_mod function, so this PR changes it to do that.

[rust-highfive]

r? @ollie27

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[jyn514]

@bors r+

[bors]

📌 Commit 214ad2f has been approved by jyn514