win: Make v8 build with /Zc:twoPhase, and improve a comment.

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}

src/zone/zone-allocator.h

@@ -26,8 +26,18 @@ class ZoneAllocator {
     using other = ZoneAllocator<O>;
   };
 
-#ifdef V8_CC_MSVC
-  // MSVS unfortunately requires the default constructor to be defined.
+#ifdef V8_OS_WIN
+  // The exported class ParallelMove derives from ZoneVector, which derives
+  // from std::vector.  On Windows, the semantics of dllexport mean that
+  // a class's superclasses that are not explicitly exported themselves get
+  // implicitly exported together with the subclass, and exporting a class
+  // exports all its functions -- including the std::vector() constructors
+  // that don't take an explicit allocator argument, which in turn reference
+  // the vector allocator's default constructor. So this constructor needs
+  // to exist for linking purposes, even if it's never called.
+  // Other fixes would be to disallow subclasses of ZoneVector (etc) to be
+  // exported, or using composition instead of inheritance for either
+  // ZoneVector and friends or for ParallelMove.
   ZoneAllocator() : ZoneAllocator(nullptr) { UNREACHABLE(); }
 #endif
   explicit ZoneAllocator(Zone* zone) : zone_(zone) {}
@@ -84,13 +94,6 @@ class RecyclingZoneAllocator : public ZoneAllocator<T> {
     using other = RecyclingZoneAllocator<O>;
   };
 
-#ifdef V8_CC_MSVC
-  // MSVS unfortunately requires the default constructor to be defined.
-  RecyclingZoneAllocator()
-      : ZoneAllocator(nullptr, nullptr), free_list_(nullptr) {
-    UNREACHABLE();
-  }
-#endif
   explicit RecyclingZoneAllocator(Zone* zone)
       : ZoneAllocator<T>(zone), free_list_(nullptr) {}
   template <typename U>