Add an additional guard to only enable std::launder aliasing if launder is actually enabled

Summary: The nightlies for MSVC included support for std::launder, but the version that released with VS2017.7 Preview 2 doesn't actually support it. The version included in the preview also adds a macro to detect if it's been enabled, so use that.

Reviewed By: yfeldblum

Differential Revision: D7388311

fbshipit-source-id: 197230c9947cb271d6107d0cd5e905643e5589d1

folly/lang/Launder.h

@@ -26,7 +26,7 @@
  *  * std::launder
  */
 
-#if __cpp_lib_launder >= 201606 || _MSC_FULL_VER >= 191426310
+#if __cpp_lib_launder >= 201606 || (_MSC_VER && _HAS_LAUNDER)
 
 namespace folly {