Alternative approach to https://codereview.chromium.org/2667983004/, which
does not depend on implicit control flow changes from
https://codereview.chromium.org/2664083002

- Remove handling for `async function` from Parser::RewriteReturn(). This functionality
is moved to BytecodeGenerator::BuildAsyncReturn(). This ensures that promise resolution
is deferred until all finally blocks are evaluated fully.

- Add a new deferred command (CMD_ASYNC_RETURN), which instructs ControlScope to
generate return code using BuildAsyncReturn rather than BuildReturn.

- Parser has a new `NewReturnStatement()` helper which determines what type of return
statement to generate based on the type of function.

BUG=v8:5896, v8:4483
R=littledan@chromium.org, neis@chromium.org, rmcilroy@chromium.org, adamk@chromium.org, gsathya@chromium.org

Review-Url: https://codereview.chromium.org/2685683002
Cr-Commit-Position: refs/heads/master@{#43104}