Add reproduction case

Author: Lucccyo

tests/test-dirs/issue1980.t

@@ -0,0 +1,32 @@
+  $ cat > test.ml <<EOF
+  > module rec M : sig
+  >   val f : unit -> unit
+  > end = struct
+  >   let f () = ()
+  > end
+  > and N : sig
+  >   val foo : unit -> unit
+  > end = struct
+  >   let foo () = M.f ()
+  > end
+  > let foo () = M.f ()
+  > EOF
+
+  $ ocamlmerlin single locate -position 11:16 -look-for implementation -filename test.ml < test.ml | jq .value
+  {
+    "file": "$TESTCASE_ROOT/test.ml",
+    "pos": {
+      "line": 4,
+      "col": 6
+    }
+  }
+
+-- Merlin should return the definition position instead of the declaration one
+  $ ocamlmerlin single locate -position 9:18 -look-for implementation -filename test.ml < test.ml | jq .value
+  {
+    "file": "$TESTCASE_ROOT/test.ml",
+    "pos": {
+      "line": 2,
+      "col": 6
+    }
+  }