fixup! Replace file_util.{h,cpp} by std::filesystem

Author: tautschnig

src/util/source_location.cpp

@@ -37,8 +37,10 @@ std::string source_locationt::as_string(bool print_cwd) const
       dest+=' ';
     dest+="file ";
     if(print_cwd)
+    {
       dest+=
-        std::filesystem::path(id2string(get_working_directory())).append(id2string(file));
+        std::filesystem::path(id2string(get_working_directory())).append(id2string(file)).string();
+    }
     else
       dest+=id2string(file);
   }
@@ -89,7 +91,7 @@ optionalt<std::string> source_locationt::full_path() const
   if(file.empty() || is_built_in(file))
     return {};
 
-  return std::filesystem::path(id2string(get_working_directory())).append(file);
+  return std::filesystem::path(id2string(get_working_directory())).append(file).string();
 }
 
 std::ostream &operator << (