Fix libc++ bug #20039: 'Constructing std::function from empty compatible std::function results in half-empty state' Thanks to Agustin Berge for the report, and for his and Eric Fiselier's work on a fix.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@212070 91177308-0d34-0410-b5e6-96231b3b80d8