[demangler] Make OutputBuffer non-copyable

[urnathan]

Summary

In addressing the buffer ownership API, I discovered a rogue member function that returned by value rather than by reference. It clearly intended to return by reference, but because the copy ctor wasn't deleted this wasn't caught.

It is not necessary to make this a move-only type, although that would be an alternative.

[bruno]

LGTM

libcxxabi/src/demangle/Utility.h

OutputBuffer &writeUnsigned(uint64_t N, bool isNeg = false) {
 
     return operator+=(StringView(TempPtr, Temp.data() + Temp.size()));
   }
 
 public:
   OutputBuffer(char *StartBuf, size_t Size)
       : Buffer(StartBuf), CurrentPosition(0), BufferCapacity(Size) {}
   OutputBuffer() = default;
+  // Non-copyable
+  OutputBuffer(const OutputBuffer &) = delete;
+  OutputBuffer &operator=(const OutputBuffer &) = delete;
+
   void reset(char *Buffer_, size_t BufferCapacity_) {
     CurrentPosition = 0;
     Buffer = Buffer_;
     BufferCapacity = BufferCapacity_;
   }
 
   /// If a ParameterPackExpansion (or similar type) is encountered, the offset
   /// into the pack that we're currently printing.
public:
   }
 
   OutputBuffer &operator+=(char C) {
     grow(1);
     Buffer[CurrentPosition++] = C;
     return *this;
   }
 
-  OutputBuffer prepend(StringView R) {
+  OutputBuffer &prepend(StringView R) {
     size_t Size = R.size();
 
     grow(Size);
     std::memmove(Buffer + Size, Buffer, CurrentPosition);
     std::memcpy(Buffer, R.begin(), Size);
     CurrentPosition += Size;
 
     return *this;

llvm/include/llvm/Demangle/Utility.h

OutputBuffer &writeUnsigned(uint64_t N, bool isNeg = false) {
 
     return operator+=(StringView(TempPtr, Temp.data() + Temp.size()));
   }
 
 public:
   OutputBuffer(char *StartBuf, size_t Size)
       : Buffer(StartBuf), CurrentPosition(0), BufferCapacity(Size) {}
   OutputBuffer() = default;
+  // Non-copyable
+  OutputBuffer(const OutputBuffer &) = delete;
+  OutputBuffer &operator=(const OutputBuffer &) = delete;
+
   void reset(char *Buffer_, size_t BufferCapacity_) {
     CurrentPosition = 0;
     Buffer = Buffer_;
     BufferCapacity = BufferCapacity_;
   }
 
   /// If a ParameterPackExpansion (or similar type) is encountered, the offset
   /// into the pack that we're currently printing.
public:
   }
 
   OutputBuffer &operator+=(char C) {
     grow(1);
     Buffer[CurrentPosition++] = C;
     return *this;
   }
 
-  OutputBuffer prepend(StringView R) {
+  OutputBuffer &prepend(StringView R) {
     size_t Size = R.size();
 
     grow(Size);
     std::memmove(Buffer + Size, Buffer, CurrentPosition);
     std::memcpy(Buffer, R.begin(), Size);
     CurrentPosition += Size;
 
     return *this;