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/Error.cpp b/llvm/lib/Support/Error.cpp
index fe8ffd8..bb02c03 100644
--- a/llvm/lib/Support/Error.cpp
+++ b/llvm/lib/Support/Error.cpp
@@ -54,7 +54,7 @@
 char ECError::ID = 0;
 char StringError::ID = 0;
 
-void logAllUnhandledErrors(Error E, raw_ostream &OS, const Twine &ErrorBanner) {
+void logAllUnhandledErrors(Error E, raw_ostream &OS, Twine ErrorBanner) {
   if (!E)
     return;
   OS << ErrorBanner;