Add support for empty values (i.e. set behaviour) and heterogeneous
lookup (lookup with a different key than the one you'll insert) to
TemplateHashMap, and use it for the string table in AstValueFactory.

Change-Id: I0c1487c9598127aac97059d4b9220e5c3c6283ce
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2494705
Auto-Submit: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Igor Sheludko <ishell@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70765}

Adds template parameters for the TemplateHashMapImpl for the key and
value type, to allow them to be something other than pointers. To keep
the impact of this patch low, uses of TemplateHashMapImpl set these
types to void* to emulate the previous behaviour.

This is part of a wider set of changes discussed in:
https://groups.google.com/forum/#!topic/v8-dev/QLsC0XPYLeM

Review-Url: https://codereview.chromium.org/2343123002
Cr-Commit-Position: refs/heads/master@{#39530}