Partially Revert "Workaround GCC bug PR78489 - SFINAE order is not respected."

This partially reverts commit r328261. The GCC bug has been fixed in
trunk and has never existed in a released version. Therefore the changes
to variant are unneeded.

However, the additional tests have been left in place.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@328388 91177308-0d34-0410-b5e6-96231b3b80d8