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.

llvm-svn: 315536
diff --git a/llvm/lib/Support/Twine.cpp b/llvm/lib/Support/Twine.cpp
index bf434f3..d17cd4e 100644
--- a/llvm/lib/Support/Twine.cpp
+++ b/llvm/lib/Support/Twine.cpp
@@ -120,10 +120,12 @@
        << Ptr.cString << "\"";
     break;
   case Twine::StdStringKind:
-    OS << "std::string:\"" << *Ptr.stdString << "\"";
+    OS << "std::string:\""
+       << Ptr.stdString << "\"";
     break;
   case Twine::StringRefKind:
-    OS << "stringref:\"" << *Ptr.stringRef << "\"";
+    OS << "stringref:\""
+       << Ptr.stringRef << "\"";
     break;
   case Twine::SmallStringKind:
     OS << "smallstring:\"" << *Ptr.smallString << "\"";