Use fast comparison against kw::Empty

We think `.as_str().lines().next().is_none()` should be equivalent to
`== kw::Empty`.

Co-authored-by: Joshua Nelson <github@jyn.dev>

src/librustdoc/passes/unindent_comments.rs

@@ -1,5 +1,7 @@
 use std::cmp;
 
+use rustc_span::symbol::kw;
+
 use crate::clean::{self, DocFragment, DocFragmentKind, Item};
 use crate::core::DocContext;
 use crate::fold::{self, DocFolder};
@@ -87,7 +89,7 @@ fn unindent_fragments(docs: &mut Vec<DocFragment>) {
     };
 
     for fragment in docs {
-        if fragment.doc.as_str().lines().next().is_none() {
+        if fragment.doc == kw::Empty {
             continue;
         }