Allow #[expect] and #[allow] within function bodies for missing_panics_doc

[Alexendoo]

Implements #11436 (comment)

[...] I'd really like to be able to (reusing some above examples),

/// Do something
pub fn string_from_byte_stream() -> String {
    let bytes = get_valid_utf8();
    #[expect(clippy::missing_panics_doc_ok, reason="caller can't do anything about this")]
    String::from_utf8(bytes).expect("`get_valid_utf8()` always returns valid UTF-8")
}

Also fixes an issue where if the first panic is in a const context disables the lint for the rest of the function

The first commit is just moving code around

changelog: [missing_panics_doc]: #[allow] and #[expect] can now be used within the function body to ignore individual panics

[rustbot]

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[duelafn]

Fantastic! You're my hero! -- Thank you!

[Jarcho]

for_each_expr could be used instead of writing a visitor since it already ignores anon constants. Should probably also search until the end to fulfill all expectations. e.g.

fn foo(x: Option<u32>, y: Option<u32>) {
    let x = x.unwrap();
    #[expect(clippy::missing_panics_doc)]
    let y = y.unwrap();
}

[Jarcho]

Thank you.