Correctly indent skipped-over code

Closes rust-lang#4398

Author: ayazhafiz

src/missed_spans.rs

@@ -353,6 +353,7 @@ impl<'a> FmtVisitor<'a> {
 
         let remaining = snippet[status.line_start..subslice.len() + offset].trim();
         if !remaining.is_empty() {
+            self.push_str(&self.block_indent.to_string(self.config));
             self.push_str(remaining);
             status.line_start = subslice.len() + offset;
         }

tests/source/issue-4398.rs

@@ -0,0 +1,19 @@
+impl Struct {
+    /// Documentation for `foo`
+    #[rustfmt::skip] // comment on why use a skip here
+    pub fn foo(&self) {}
+}
+
+impl Struct {
+    /// Documentation for `foo`
+       #[rustfmt::skip] // comment on why use a skip here
+    pub fn foo(&self) {}
+}
+
+/// Documentation for `Struct`
+#[rustfmt::skip] // comment
+impl Struct {
+    /// Documentation for `foo`
+       #[rustfmt::skip] // comment on why use a skip here
+    pub fn foo(&self) {}
+}

tests/target/issue-4398.rs

@@ -0,0 +1,19 @@
+impl Struct {
+    /// Documentation for `foo`
+    #[rustfmt::skip] // comment on why use a skip here
+    pub fn foo(&self) {}
+}
+
+impl Struct {
+    /// Documentation for `foo`
+    #[rustfmt::skip] // comment on why use a skip here
+    pub fn foo(&self) {}
+}
+
+/// Documentation for `Struct`
+#[rustfmt::skip] // comment
+impl Struct {
+    /// Documentation for `foo`
+       #[rustfmt::skip] // comment on why use a skip here
+    pub fn foo(&self) {}
+}