C++: add support for C++17 fold expressions

cpp/ql/src/semmlecode.cpp.dbscheme

@@ -1432,6 +1432,7 @@ case @expr.kind of
 | 129 = @new_array_expr
 // ... 130 @objc_array_literal deprecated
 // ... 131 @objc_dictionary_literal deprecated
+| 132 = @foldexpr
 // ...
 | 200 = @ctordirectinit
 | 201 = @ctorvirtualinit
@@ -1579,6 +1580,12 @@ lambda_capture(
 @addressable = @function | @variable ;
 @accessible = @addressable | @enumconstant ;
 
+fold(
+    int expr: @foldexpr ref,
+    string operator: string ref,
+    boolean is_left_fold: boolean ref
+);
+
 stmts(
     unique int id: @stmt,
     int kind: int ref,