Add ability to strip doc / attributes info from the Span

[Mingun]

I'm working on and PR for fixing https://github.com/serde-rs/serde/issues/2105 and the related stuff and making compiletests for them. For the piece

#[derive(Serialize)]
#[serde(tag = "type")]
enum Serializable {
    /// Error should be reported
    Tuple2(u8, u8),
}

the error span would cover several lines:

error: #[serde(tag = "...")] cannot be used with tuple variants
  --> tests/ui/enum-representation/internal-tuple-variant.rs:10:5
   |
10 | /     /// Error should be reported
11 | |     Tuple2(u8, u8),
   | |__________________^

Because doc comments can be very long, it is impractical to bring them into error messages. I would like to get the following message:

error: #[serde(tag = "...")] cannot be used with tuple variants
  --> tests/ui/enum-representation/internal-tuple-variant.rs:10:5
   |
10 |       /// Error should be reported
11 |       Tuple2(u8, u8),
   |       ^^^^^^^^^^^^^^

That error is generated by the following code in serde (here in action):

// variant.original is a `&syn::Variant`
cx.error_spanned_by(
    variant.original,
    "#[serde(tag = \"...\")] cannot be used with tuple variants",
);

Is there any way to strip span that related to doc comments (and probably to all other attributes) and keep only code span of syn::Variant, syn::Field, etc?