flambda-backend: Fix labeled tuple pattern location in parser, again. (#2637)

Author: ccasin

parsing/parser.mly

@@ -3529,10 +3529,12 @@ pattern_no_exn:
       { let loc = $loc(label) in
         Some label, mkpatvar ~loc label }
   | TILDE LPAREN label = LIDENT COLON cty = core_type RPAREN %prec COMMA
-      { let loc = $loc(label) in
-        let pat = mkpatvar ~loc label in
-        Some label, mkpat_opt_constraint ~loc pat (Some cty) }
+      { let lbl_loc = $loc(label) in
+        let pat_loc = $startpos($2), $endpos in
+        let pat = mkpatvar ~loc:lbl_loc label in
+        Some label, mkpat_opt_constraint ~loc:pat_loc pat (Some cty) }
 
+(* If changing this, don't forget to change its copy just above. *)
 %inline labeled_tuple_pat_element_noprec(self):
   | self { None, $1 }
   | LABEL simple_pattern