Ignore manual_map clippy lint

I do not find that it makes the code clearer in this case, given the
related `if` branch right above.

    error: manual implementation of `Option::map`
       --> impl/src/ast.rs:145:16
        |
    145 |           } else if let Some(transparent) = &self.transparent {
        |  ________________^
    146 | |             Some(transparent.span)
    147 | |         } else {
    148 | |             None
    149 | |         }
        | |_________^ help: try this: `self.transparent.as_ref().map(|transparent| transparent.span)`
        |
        = note: `-D clippy::manual-map` implied by `-D clippy::all`
        = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#manual_map

impl/src/lib.rs

@@ -1,6 +1,7 @@
 #![allow(
     clippy::blocks_in_if_conditions,
     clippy::cast_possible_truncation,
+    clippy::manual_map,
     clippy::map_unwrap_or,
     clippy::needless_pass_by_value,
     clippy::option_if_let_else,