This patch removes a few unnecessary bits of async function
parsing (the PrepareAsyncFunctionBody() helper method, the
FunctionBodyType enum) by doing separate handling of
block and single-expression async arrow functions.

Change-Id: I64f837635a23eaf06d42887ca7f9ac59c768f0f2
Reviewed-on: https://chromium-review.googlesource.com/601247
Commit-Queue: Adam Klein <adamk@chromium.org>
Reviewed-by: Marja Hölttä <marja@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47169}