Object: Provide a richer means of describing auxiliary symbols

The current state of affairs has auxiliary symbols described as a big
bag of bytes. This is less than satisfying, it detracts from the YAML
file as being human readable.

Instead, allow for symbols to optionally contain their auxiliary data.
This allows us to have a much higher level way of describing things like
weak symbols, function definitions and section definitions.

This depends on D3105.

Differential Revision: http://llvm-reviews.chandlerc.com/D3092

llvm-svn: 204214