This assignment operator was previously broken since the SFINAE always resulted
in substitution failure. This caused assignments to turn into
copy construction + assignment.
This patch was originally committed as r279953 but was reverted due to warnings
in the test-suite. This new patch corrects those warnings.
git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@279955 91177308-0d34-0410-b5e6-96231b3b80d8