Fix edge case where // inside a sequence wrapped in braces didn't break.

```reason
let x = {
  //
};
```

If the { }-sequence didn't break we would get parse error:

```reason
let x = { // };
```

Author: IwanKaramazow

formatTest/unit_tests/expected_output/lineComments.re

@@ -353,6 +353,10 @@ let tuple_equal = ((csu, mgd)) =>
  */
 let fun_def_comment_inline = () => {/* */};
 
+let fun_def_comment_newline = () => {
+  //
+};
+
 let fun_def_comment_long = () => {
   /* longer comment inside empty function body */
 };

formatTest/unit_tests/input/lineComments.re

@@ -361,6 +361,10 @@ let tuple_equal((csu, mgd)) =
  */
 let fun_def_comment_inline = () => { /* */ };
 
+let fun_def_comment_newline = () => {
+  //
+};
+
 let fun_def_comment_long = () => { /* longer comment inside empty function body */};
 
 let trueThing = true;

src/reason-parser/reason_comment.ml

@@ -48,3 +48,7 @@ let is_doc t =
 
 let make ~location category text =
   { text; category; location }
+
+let isLineComment {category; text} = match category with
+  | SingleLine -> Reason_syntax_util.isLineComment text
+  | EndOfLine | Regular -> false

src/reason-parser/reason_pprint_ast.ml

@@ -1487,7 +1487,22 @@ let rec insertSingleLineComment layout comment =
   | Easy _ ->
     prependSingleLineComment comment layout
   | Sequence (listConfig, subLayouts) when subLayouts == [] ->
-    (* If there are no subLayouts (empty body), create a Sequence of just the comment *)
+    (* If there are no subLayouts (empty body), create a Sequence of just the
+     * comment. We need to be careful when the empty body contains a //-style
+     * comment. Example:
+     *   let make = () => {
+     *     //
+     *   };
+     * It is clear that the sequence needs to always break here, otherwise
+     * we get a parse error: let make = () => { // };
+     * The closing brace and semicolon `};` would become part of the comment…
+     *)
+    let listConfig =
+      if Reason_comment.isLineComment comment then
+        {listConfig with break = Always_rec}
+      else
+        listConfig
+    in
     Sequence (listConfig, [formatComment comment])
   | Sequence (listConfig, subLayouts) ->
     let (beforeComment, afterComment) =