Handle multiple error fix suggestions carefuly

The existing code seems to assume that substitutions spans are disjoint,
which is not always the case.

In the example:

    pub trait AAAA {}
    pub trait B {}
    pub trait C {}
    pub type T<P: AAAA + B + C> = P;

, we get three substituions starting from ':' and ending respectively at
the end of each trait token.

With the former offset calculation, this would cause `underline_start` to
eventually become negative before being converted to `usize`...

The new version may report erroneous results for non perfectly overlapping
substitutions but I don't know if such examples exist. Alternatively, we
could detect these cases and trim out overlapping substitutions.

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