[filepath] add is_unknown function

src/libraries/utils/filepath.ml

@@ -224,6 +224,7 @@ module Normalized = struct
   let pretty fmt p = Format.fprintf fmt "%s" (pretty p)
   let pp_abs fmt p = Format.fprintf fmt "%s" p
   let unknown = normalize ""
+  let is_unknown fp = equal fp unknown
 end
 
 type position =

src/libraries/utils/filepath.mli

@@ -120,6 +120,9 @@ module Normalized: sig
 
   (** Unknown filepath, used as 'dummy' for [Datatype.Filepath]. *)
   val unknown: t
+
+  (** @since Frama-C+dev *)
+  val is_unknown: t -> bool
 end
 
 (** Describes a position in a source file.