This adds support for union types to Torque.

There is a new type expression
A | B
to form the union of the type expressions A and B.
This is only possible if A and B have a common supertype, to prevent
nonsensical unions of types with different representations.

Union types are normalized:
A | B == B | A
A | (B | C) == (A | B) | C
A | A == A

The subtyping rules are defined recursively:
(A | B) <: C  if  A <: C and B <: C
A <: (B | C)  if  A <: B or A <: C

This allows to define Object as a union type:

type Tagged generates 'TNode<Object>';
type Smi extends Tagged generates 'TNode<Smi>';
type HeapObject extends Tagged generates 'TNode<HeapObject>';
type Object = Smi | HeapObject;

The type {Tagged} is introduced to have a common supertype of all
tagged values, but we should not use it directly, because {Object}
contains the additional information that there is nothing but {Smi}
and {HeapObject} values.

When mapping union types to CSA types, we select the most specific
common supertype. For Number and Numeric, we already use union types
on the CSA side. Since it is not possible to map to CSA union types
in general, we special-case these two union types to map them to
the CSA union types we already use.

Bug: v8:7793
Change-Id: I7a4e466436f55d04012f29ef17acfdb957653908
Reviewed-on: https://chromium-review.googlesource.com/1076132Reviewed-by: Michael Stanton <mvstanton@chromium.org>
Commit-Queue: Tobias Tebbi <tebbi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53411}