This CL introduces generic Torque structs. Generics are grounded early in the Torque compilation pipeline, meaning that every instantiation of a generic struct with concrete types will be turned into a distinct StructType.

As an example, consider a Tuple of types T1, T2:

  struct Tuple<T1: type, T2: type> {
    const fst: T1;
    const snd: T2;
  }

which can be manipulated using generic macros, such as

  macro Swap<T1: type, T2: type>(tuple: Tuple<T1, T2>): Tuple<T2, T1> {
    return Tuple<T2, T1>{fst: tuple.snd, snd: tuple.fst};
  }

Currently there is no type inference for struct instantiation sites, so type arguments have to be provided explicitly:

  const intptrAndSmi = Tuple<intptr, Smi>{fst: 1, snd: 2};

R=sigurds@chromium.org, tebbi@chromium.org

Change-Id: I43111561cbe53144db473dc844a478045644ef6c
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1714868
Commit-Queue: Georg Schmid <gsps@google.com>
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
Cr-Commit-Position: refs/heads/master@{#62878}