Split definition of kMscVer from kIsWindows

Summary: [Folly] Split definition of `kMscVer` from `kIsWindows` and use in `hazptr`.

Reviewed By: mzlee

Differential Revision: D7027159

fbshipit-source-id: 1dd2bad821919a2d579aa615ecb2921e5a3ce4d8

folly/Portability.h

@@ -341,9 +341,13 @@ constexpr auto kIsLinux = false;
 
 #if defined(_WIN32)
 constexpr auto kIsWindows = true;
-constexpr auto kMscVer = _MSC_VER;
 #else
 constexpr auto kIsWindows = false;
+#endif
+
+#if _MSC_VER
+constexpr auto kMscVer = _MSC_VER;
+#else
 constexpr auto kMscVer = 0;
 #endif
 } // namespace folly

folly/experimental/hazptr/hazptr-impl.h

@@ -314,7 +314,7 @@ class hazptr_priv {
 };
 
 static_assert(
-    folly::kIsWindows || std::is_trivial<hazptr_priv>::value,
+    folly::kMscVer || std::is_trivial<hazptr_priv>::value,
     "hazptr_priv must be trivial to avoid a branch to check initialization");
 
 void hazptr_priv_init();