Gate hazptr static_assert on _CPPLIB_VER vs _MSC_VER

Summary:
[Folly] Gate `hazptr` `static_assert` on `_CPPLIB_VER` vs `_MSC_VER`.

It's a property of the standard library implementation, not a property of the compiler front-end.

Reviewed By: smeenai, Orvid

Differential Revision: D7378222

fbshipit-source-id: 78d04252df751b683c0099cb0c273302ff388b73

folly/Portability.h

@@ -363,6 +363,14 @@ constexpr auto kIsGlib49 = true;
 #else
 constexpr auto kIsGlib49 = false;
 #endif
+
+// cpplib is an implementation of the standard library, and is the one typically
+// used with the msvc compiler
+#if _CPPLIB_VER
+constexpr auto kCpplibVer = _CPPLIB_VER;
+#else
+constexpr auto kCpplibVer = 0;
+#endif
 } // namespace folly
 
 // Define FOLLY_USE_CPP14_CONSTEXPR to be true if the compiler's C++14

folly/experimental/hazptr/hazptr-impl.h

@@ -315,7 +315,7 @@ class hazptr_priv {
 };
 
 static_assert(
-    folly::kMscVer || std::is_trivial<hazptr_priv>::value,
+    folly::kCpplibVer || std::is_trivial<hazptr_priv>::value,
     "hazptr_priv must be trivial to avoid a branch to check initialization");
 
 void hazptr_priv_init(hazptr_priv& priv);