This JSAsyncFunctionObject represents the implicit generator object
inside of async functions, and also holds the outer promise for the
async functions. This in turn allows us to get rid of the .promise
in the Parser / BytecodeGenerator completely, and will make it
possible to build zero-cost async stack traces independent of the
concrete synchronous part of the stack frame (which currently breaks
in Node.js).

In the bytecode all the async function operations now take this new
JSAsyncFunctionObject instead of passing both the .generator_object
and the .promise, which further simplifies and shrinks the bytecode.
It also reduces the size of async function frames, potentially making
the suspend/resume cheaper.

This also changes `await` to use intrinsics instead of calling to
special JSFunctions on the native context, and thus reduces the size of
the native contexts.

Drive-by-fix: Introduce a dedicated JSCreateAsyncFunctionObject operator
to TurboFan.

Bug: v8:7253, v8:7522
Change-Id: I2305302285156aa1f71328ecac70377abdd92c80
Ref: nodejs/node#11865
Design-Document: http://bit.ly/v8-zero-cost-async-stack-traces
Reviewed-on: https://chromium-review.googlesource.com/c/1273049
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Reviewed-by: Sathya Gunasekaran <gsathya@chromium.org>
Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Reviewed-by: Maya Lekova <mslekova@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56554}