Revert "[ADT] Make Twine's copy constructor private."

This reverts commit 4e4ee1c507e2707bb3c208e1e1b6551c3015cbf5.

This is failing due to some code that isn't built on MSVC
so I didn't catch.  Not immediately obvious how to fix this
at first glance, so I'm reverting for now.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@315536 91177308-0d34-0410-b5e6-96231b3b80d8
17 files changed