In the process:
- Add a command-line flag --opt-code-positions to track source position information throughout optimized code.
- Add a subclass of the hydrogen graph builder to ensure that the source position is properly set on the graph builder for all generated hydrogen code.
- Overhaul handling of source positions in hydrogen to ensure they are passed through to generated code consistently and in most cases transparently.

Originally reviewed in this CL: https://codereview.chromium.org/24957003/

Review URL: https://codereview.chromium.org/29123008

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@17295 ce2b1a6d-e550-0410-aec6-3dcde31c8c00