Allow specifying experimental sanitizers.

[oliverchang]

See #378.

[inferno-chromium]

LGTM

[mikea]

Not sure why would you want to do this. Why not simply comments in project.yaml?

sanitizers:
  - undefined # experimental

[oliverchang]

it's much easier to parse. I don't think yaml parser preserves comments?

[oliverchang]

please take another look. Changed to support:

sanitizers:
  - address
  - undefined:
      experimental: true

In addition to what we already support (list of sanitizer strings)

[oliverchang]

Thanks. Will merge this next week, since Mike is testing builder changes.