Improve layout history error message: hide layout history reason of various generalized vars

[alanechang]

This PR erases the layout history reason of certain generalized type variables. Motivation behind this is when explaining the reason of an error, the implementation details of some other value/type defined far away is not particularly interesting nor helpful to the user.

To illustrate, this PR changes the error message of this example:

module _ = struct
  module M : sig
    val f : 'a -> 'a
  end = struct
    let f x = x
  end
  let g (x : t_void) = M.f x
end

in such a way:

Line 8, characters 27-28:
8 |   let g (x : t_void) = M.f x
                               ^
Error: This expression has type t_void but an expression was expected of type
         ('a : value)
       The layout of t_void is void, because
         of the annotation on the declaration of the type t_void.
       But the layout of t_void must be a sublayout of value, because
-        it's used as a function argument, defaulted to layout value.
+        of definition of f at Line 3, characters 4-20.

This also puts the error reporting of layouts more in line with what's being done for types.

In terms of implementation details, this layout reason erasing behavior is added at three places:

• for let statements within type_let
• for val/external declarations within transl_value_decl
• for only the type params of type declarations within transl_type_decl