With the changes in this patch,	it is now possible to add methods to
both Torque's class and struct types. As a special case, "constructor"
methods are used to initialize the values of classes and structs when
they are constructed.

The functionality in this patch	includes:

- The refactoring of class- and struct-handling code to share field
  and method declaration code between both.

- Addition of the "%Allocate" intrinsic that allocates raw bytes to be
  allocated from the V8 GC's NewSpace heap as the basis for freshly
  created, initialized class objects.

- An implementation of a CallMethodExpression AST node that enables
  calling methods and constructors, including special handling of
  passing through the "this" pointer for method calls on structs by
  reference. The syntax for struct construction using "{}" remains as
  before, but now calls the struct's matching constructor rather than
  implicitly initializing the struct fields with the initialization
  arguments. A new syntax for allocation classes is introduced: "new
  ClassName{constructor_param1, constructor_param1, ...}", which
  de-sugars to an %Allocate call followed by a call to the matching
  constructor.

- class	constructors can use the "super" keyword to initialize	their
  super class.

- If classes and struct do not have a constructor, Torque creates a
  default constructor for them based on their field declarations,
  where each field's initial value is assigned to a same-typed
  parameter to the the default constructor. The default constructor's
  parameters are in field-declaration order, and for derived classes,
  the default constructor automatically uses a "super" initialization
  call to initialize inherited fields.

- Class field declarations now automatically create ".field" and
  ".field=" operators that create CSA-compatible object accessors.

- Addition of a no-argument constructor for JSArrays that creates an
  empty, PACKED_SMI_ELEMENTS JSArray using the machinery added
  elsewhere in this patch.

Bug: v8:7793
Change-Id: I31ce5f4b444656ab999555d780aeeba605666bfa
Reviewed-on: https://chromium-review.googlesource.com/c/1392192
Commit-Queue: Daniel Clifford <danno@chromium.org>
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58860}