This code change allows users to use --enable-error-code all to enable all error codes. This "fixes" #2376 (which was closed as not planned — but a decade of hindsight has, I think, revealed that the idea for a flag like this is more practical than it initially seemed). It also addresses a feature request that was mentioned in the course of discussing #18078 (it does not fix #18078, which is an issue about something else).

My main motivation to add this feature is that it's very easy to add and somewhat useful to me and others, who sometimes want to see what the effect of enabling all error codes is, whether we decide to check our code with that every time or just want to evaluate all of the codes at once to see which are worthwhile.

I don't think this PR closes further discussion on whether, eg, --strict should also enable every error code, which I also think should be true, but for other reasons; nor what flags should control what other error codes, which is also a fertile grounds for discussion. I conceptualize --enable-error-codes all as sort of a "low-level" command line option that does something simple to define (all), whereas other flags do something more ergonomic like give you some recommended set of error codes.

This code change seems like it should allow not just command line usage but also configuration file usage, but for some opaque reason (perhaps my machine configuration is just cursed?) this did not work for me (it resulted in an "invalid error code" error message). So it's documented as only a command line feature.

The test for this feature spotchecks that --enable-error-code all enabled explicit-override and a couple other errors.