Template objects should be cached after they are first created and reused on
subsiquent calls to tag functions. Currently these cached objects are stored
on the feedback vector, which has appropriate lifetime, however with bytecode
flushing the feedback vector could be cleared when the bytecode is flushed,
causing the template object to be dropped.

In order to retain the cached template objects in the face of bytecode flushing,
this CL adds a weakmap for each native context that is (weakly) keyed by
shared function info, and holds a linked list of cached template objects
associated with that shared function info, indexed by feedback vector slot id.
Misses will check this weakmap, and if no entry is found, a new template object
is created and added into this weakmap alongside the feedback vector.

BUG=v8:8799,v8:8799,v8:8395

Change-Id: Ia95d5cfc394ce58dc9fe6a1e49780f05299acc17
Reviewed-on: https://chromium-review.googlesource.com/c/1477746
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59818}