Add new ubegin/uend API to StringRef/MDString

[loladiro]

Summary

In some instances (such as in BitCodeWriter - see http://reviews.llvm.org/D6184),
it is useful to get an unsiged version of the data pointer instead. This adds an
appropriate API to be used in BitCodeWriter in a follow up commit.

[dblaikie]

Could we just do this as a range API without begin/end? Or maybe ArrayRef<>?

It'd be helpful to see the use case to know what a good design is here.

[loladiro]

This was suggested in http://reviews.llvm.org/D6184.

[loladiro]

Address Review comments

include/llvm/ADT/StringRef.h

namespace llvm {
   ///
   /// This class does not own the string data, it is expected to be used in
   /// situations where the character data resides in some other buffer, whose
   /// lifetime extends past that of the StringRef. For this reason, it is not in
   /// general safe to store a StringRef.
   class StringRef {
   public:
     typedef const char *iterator;
+    typedef const unsigned char *uiterator;
     typedef const char *const_iterator;
     static const size_t npos = ~size_t(0);
     typedef size_t size_type;
 
   private:
     /// The start of the string, in an external buffer.
     const char *Data;
 
public:
     /*implicit*/ StringRef(const std::string &Str)
       : Data(Str.data()), Length(Str.length()) {}
 
     /// @}
     /// @name Iterators
     /// @{
 
     iterator begin() const { return Data; }
+    uiterator ubegin() const { return reinterpret_cast<uiterator>(begin()); }
 
     iterator end() const { return Data + Length; }
+    uiterator uend() const { return reinterpret_cast<uiterator>(end()); }
 
     /// @}
     /// @name String Operations
     /// @{
 
     /// data - Get a pointer to the start of the string (which may not be null
     /// terminated).
     const char *data() const { return Data; }

include/llvm/IR/Metadata.h

static MDString *get(LLVMContext &Context, const char *Str) {
     return get(Context, Str ? StringRef(Str) : StringRef());
   }
 
   StringRef getString() const { return getName(); }
 
   unsigned getLength() const { return (unsigned)getName().size(); }
 
   typedef StringRef::iterator iterator;
+  typedef StringRef::uiterator uiterator;
 
   /// \brief Pointer to the first byte of the string.
   iterator begin() const { return getName().begin(); }
+  uiterator ubegin() const { return getName().ubegin(); }
 
   /// \brief Pointer to one byte past the end of the string.
   iterator end() const { return getName().end(); }
+  uiterator uend() const { return getName().uend(); }
 
   /// \brief Methods for support type inquiry through isa, cast, and dyn_cast.
   static bool classof(const Value *V) {
     return V->getValueID() == MDStringVal;
   }
 };
 
 /// \brief A collection of metadata nodes that might be associated with a