All C++ functions called directly from generated code must have
a predictable ABI. We ensure that by requiring their return and
argument types to be scalars -- in particular, they must not be
non-pointer ObjectPtr or ObjectSlot types, which is easy to get
wrong and difficult to debug. This patch adds compile-time type
checks enforcing the requirement to the macro used for creating
ExternalReferences for functions.

Bug: v8:3770
Change-Id: I442cf25e2f72b7ea84d4a50c9c665b187b179ca0
Reviewed-on: https://chromium-review.googlesource.com/c/1334974
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57560}