[ntcore] Fix write_impl (#5847)

The previous fix didn't handle all cases correctly. Instead, add a new
function to raw_ostream (SetNumBytesInBuffer) to allow always using the
full buffer size, and revamp write_impl to more cleanly handle all
cases.

wpiutil/src/main/native/thirdparty/llvm/include/wpi/raw_ostream.h

@@ -356,6 +356,11 @@ protected:
     SetBufferAndMode(BufferStart, Size, BufferKind::ExternalBuffer);
   }
 
+  /// Force-set the number of bytes in the raw_ostream buffer.
+  void SetNumBytesInBuffer(size_t Size) {
+    OutBufCur = OutBufStart + Size;
+  }
+
   /// Return an efficient buffer size for the underlying output mechanism.
   virtual size_t preferred_buffer_size() const;