fix(engine/import-thir): fix leaking errors

The `capture` function of `diagnostics.ml` was baldy implemented.
Seems like in the expression `(f (), g ())` OCaml first runs `g` and
then `f`!

I'm using a state to capture errors, and using `(f (), !state)`, I was
ignoring the side effects of `f`, which is... the only point of that
`capture` function.

engine/lib/diagnostics.ml

@@ -121,7 +121,11 @@ end = struct
 
   let capture (type a) (f : unit -> a) : a * t list =
     let previous_state = !state in
-    let result = (f (), !state) in
+    state := [];
+    let result =
+      let x = f () in
+      (x, !state)
+    in
     state := previous_state;
     result
 end