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.h b/llvm/tools/llvm-objcopy/Object.h
index d266912..f608843 100644
--- a/llvm/tools/llvm-objcopy/Object.h
+++ b/llvm/tools/llvm-objcopy/Object.h
@@ -29,11 +29,11 @@
       : Sections(Secs) {}
   SectionTableRef(const SectionTableRef &) = default;
 
-  SectionBase *getSection(uint16_t Index, const llvm::Twine &ErrMsg);
+  SectionBase *getSection(uint16_t Index, llvm::Twine ErrMsg);
 
   template <class T>
-  T *getSectionOfType(uint16_t Index, const llvm::Twine &IndexErrMsg,
-                      const llvm::Twine &TypeErrMsg);
+  T *getSectionOfType(uint16_t Index, llvm::Twine IndexErrMsg,
+                      llvm::Twine TypeErrMsg);
 };
 
 class SectionBase {