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/tools/llvm-objcopy/llvm-objcopy.cpp b/llvm/tools/llvm-objcopy/llvm-objcopy.cpp
index 9fc2897..7f55a43 100644
--- a/llvm/tools/llvm-objcopy/llvm-objcopy.cpp
+++ b/llvm/tools/llvm-objcopy/llvm-objcopy.cpp
@@ -27,7 +27,7 @@
 
 namespace llvm {
 
-LLVM_ATTRIBUTE_NORETURN void error(const Twine &Message) {
+LLVM_ATTRIBUTE_NORETURN void error(Twine Message) {
   errs() << ToolName << ": " << Message << ".\n";
   errs().flush();
   exit(1);