add default "auto" as option to --check-bounds

[IanButterworth]

It seems to be unusual that neither of the --check-bounds={yes|no} options are the default state, so this adds "auto" to make it --check-bounds={yes|no|auto}

It would make overruling this arg back to default state possible, for instance in JuliaLang/Pkg.jl#2668 (although that can be fixed in other ways)

[IanButterworth]

Who's a good reviewer for this? @vtjnash perhaps?

[JeffBezanson]

Looks fine to me, but this is not the kind of thing we would usually backport.

[IanButterworth]

My rationale for backporting to 1.6 was that this would be helpful for julia-actions/julia-runtest#46
If it's not backported to the LTS, the user would have to guard in CI to not send --check-bounds=auto to the LTS, assuming they test on LTS and latest.
And thus couldn't run CI tests with the julia default bounds checking.

I thought the default not being available as an option was more of a bug than a missing feature? That's probably a thin argument though

[JeffBezanson]

Yeah, it's a borderline case, and indeed difficult to see this causing any problem. I guess it might be ok to backport, but I would probably not add it to the HISTORY file.

[IanButterworth]

Ok. I'll watch out for when it gets backported. I couldn't see a label for "requires manual backport". Perhaps that would help