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/Object.cpp b/llvm/tools/llvm-objcopy/Object.cpp
index f4c159c..f9acf00 100644
--- a/llvm/tools/llvm-objcopy/Object.cpp
+++ b/llvm/tools/llvm-objcopy/Object.cpp
@@ -428,15 +428,15 @@
   }
 }
 
-SectionBase *SectionTableRef::getSection(uint16_t Index, const Twine &ErrMsg) {
+SectionBase *SectionTableRef::getSection(uint16_t Index, Twine ErrMsg) {
   if (Index == SHN_UNDEF || Index > Sections.size())
     error(ErrMsg);
   return Sections[Index - 1].get();
 }
 
 template <class T>
-T *SectionTableRef::getSectionOfType(uint16_t Index, const Twine &IndexErrMsg,
-                                     const Twine &TypeErrMsg) {
+T *SectionTableRef::getSectionOfType(uint16_t Index, Twine IndexErrMsg,
+                                     Twine TypeErrMsg) {
   if (T *Sec = llvm::dyn_cast<T>(getSection(Index, IndexErrMsg)))
     return Sec;
   error(TypeErrMsg);