add test showing that locs are wrong in record type spread with overlapping labels

Author: zth

jscomp/build_tests/super_errors/expected/record_type_spreads.res.expected

@@ -0,0 +1,11 @@
+
+  We've found a bug for you!
+  /.../fixtures/record_type_spreads.res:3:12-20
+
+  1 │ type t = {x: int, y: string}
+  2 │ 
+  3 │ type t2 = {x: string, y: float}
+  4 │ 
+  5 │ type t3 = {...t, ...t2}
+
+  Two labels are named x

jscomp/build_tests/super_errors/fixtures/record_type_spreads.res

@@ -0,0 +1,5 @@
+type t = {x: int, y: string}
+
+type t2 = {x: string, y: float}
+
+type t3 = {...t, ...t2}