Replace SCOPE flags with bitfields

[dkorpel]

In light of the recent #16752, get rid of error-prone bitmasking logic.

While we're at it, I renamed some of the fields because without flags they can be misleading: sc.free sounds like it calls free(sc), so rename it to sc.canFree.

There's 2 flags which aren't booleans, but instead form an ad-hoc enumeration of contracts, so I converted that to an enum Contract and added enum support to dmd.common.bitfields. I can split that improvement off in its own PR to make this one easier to review.

I kept the layout the same as with the SCOPE flag to be sure, but those _padding fields can be removed in a next PR.

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[dkorpel]

Ping @RazvanN7