LWG 1385 [FCD] tuple_cat should be a single variadic signature...

LWG 1385 [FCD] tuple_cat should be a single variadic signature (http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#1385).  This issue is only in Ready status, meaning it is not official, but probably will be this March in Madrid.  It was tentatively accepted in Batavia with the previso that Bill and I didn't have any problems implementing it.  This is my part of that agreement.

llvm-svn: 121619