The C++ standard says that template functions should be parsed immediately,
and only type-dependent things should be deferred.

cl.exe (MSVC's compiler) instead deferred parsing of all template functions
until the end of the translation unit, and unreferenced template functions
are not parsed at all. clang-cl emulates cl.exe's behavior.

Recently, cl.exe (and clang-cl) grew a /Zc:twoPhase flag that opts in to the
standards-conforming behavior, and system headers are now clean enough
to build with this flag set.

This cleans up v8 to also build with this flag. There was just a single issue:
RecyclingZoneAllocator() is unused and contains invalid code: It calls
the superclass ctor using `ZoneAllocator(nullptr, nullptr)`, when it should
be doing `ZoneAllocator<T>(nullptr, nullptr)`. With /Zc:twoPhase, this is
now a parsing error. However, since the RecyclingZoneAllocator() default
constructor isn't used anywhere, just delete it.

Finally, improve the comment for ZoneAllocator's default constructor to
explain why it's needed on Windows.

Bug: chromium:969702
Change-Id: I7a516afde67fe090a512d7c7214a3c6932754aca
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1652503
Auto-Submit: Nico Weber <thakis@chromium.org>
Commit-Queue: Sigurd Schneider <sigurds@chromium.org>
Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
Cr-Commit-Position: refs/heads/master@{#62108}