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/Object/Error.cpp b/llvm/lib/Object/Error.cpp
index d4ab916..7d43a84 100644
--- a/llvm/lib/Object/Error.cpp
+++ b/llvm/lib/Object/Error.cpp
@@ -60,10 +60,9 @@
 char BinaryError::ID = 0;
 char GenericBinaryError::ID = 0;
 
-GenericBinaryError::GenericBinaryError(const Twine &Msg) : Msg(Msg.str()) {}
+GenericBinaryError::GenericBinaryError(Twine Msg) : Msg(Msg.str()) {}
 
-GenericBinaryError::GenericBinaryError(const Twine &Msg,
-                                       object_error ECOverride)
+GenericBinaryError::GenericBinaryError(Twine Msg, object_error ECOverride)
     : Msg(Msg.str()) {
   setErrorCode(make_error_code(ECOverride));
 }