flambda-backend: Adjust the location on as pattern vars for better errors/warnings (#1835)

This adjusts the location on as patterns for better errors/warnings

Author: ccasin

testsuite/tests/typing-gadts/or_patterns.ml

@@ -217,9 +217,9 @@ let simple_merged_annotated_return (type a) (t : a t) (a : a) =
 ;;
 
 [%%expect{|
-Line 3, characters 12-20:
+Line 3, characters 18-19:
 3 |   | IntLit, (3 as x)
-                ^^^^^^^^
+                      ^
 Error: This pattern matches values of type a
        This instance of a is ambiguous:
        it would escape the scope of its equation

testsuite/tests/warnings/w26_alias.ml

@@ -0,0 +1,19 @@
+(* TEST
+   * expect
+*)
+type t =
+  { x : int
+  ; y : int
+  }
+
+let sum ({ x; y } as t) = x + y
+
+[%%expect{|
+type t = { x : int; y : int; }
+Line 6, characters 21-22:
+6 | let sum ({ x; y } as t) = x + y
+                         ^
+Warning 26 [unused-var]: unused variable t.
+val sum : t -> int = <fun>
+|}]
+

typing/typecore.ml

@@ -2540,7 +2540,7 @@ and type_pat_aux
         let ty_var, mode = solve_Ppat_alias ~refine ~mode:alloc_mode.mode env q in
         let mode = mode_cross_to_min !env expected_ty mode in
         let id =
-          enter_variable ~is_as_variable:true tps loc name mode
+          enter_variable ~is_as_variable:true tps name.loc name mode
             ty_var sp.ppat_attributes
         in
         rvp k {