[libcxx] Remove use of default function template parameters in type traits. Fixes DR20484

Summary: This patch moves the SFINAE for __is_destructor_welformed out of the function template parameters. type_traits must compile in c++03 mode since it is included in c++03 headers.

Test Plan: No tests have been added.

Reviewers: danalbert, mclow.lists

Reviewed By: danalbert

Subscribers: K-ballo, cfe-commits

Differential Revision: http://reviews.llvm.org/D4735