[v3 PATCH] Implement P0032R3, Homogeneous interface for variant, any and optional, for the parts concerning any and optional.

[Jonathan Wakely]

This change replaced a Tab with 8 spaces, but the lines after it still
use Tabs:

@@ -641,12 +634,9 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
         }
 
       template<typename... _Args>
-	void
+        enable_if_t<is_constructible<_Tp, _Args&&...>::value>
 	emplace(_Args&&... __args)
 	{

Might as well keep it consistent (and save 7 bytes! ;)

OK for trunk, thanks.