Makes the Node* versions private so they can still be called from the
inlined TNode versions.

Also changes to LoadFromParentFrame to return TNode<Object> since its
uses don't require anything looser.

Bug: v8:10021
Change-Id: I84e3831d444a7787e0b03ff2f9e665181a9caa06
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1980578
Commit-Queue: Dan Elphick <delphick@chromium.org>
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#65604}