Disable fatal warnings in the library

[dwijnand]

The fatal warnings PR (for library) was merged prematurely as it is
impacting my exhaustivity PR (1). Rather do anything (like rebase or tweak)
to the 12 commits that my PR consists of and therefore causing CI to
rebuild each commit, I'm separately going to PR disabling fatal
warnings, so I can merge my PR without it instantly breaking the 2.13.x
branch.

I'll do the same for the reflect and compiler PRs, and once we've
re-STARR'd we can re-enable fatal warnings by either resolving all the
exhaustivity warnings or opting out of them (or a mix of both).

cc @NthPortal

[NthPortal]

I would rather have temporarily disabled exhaustivity warnings via -Wconf than disabled fatal warnings entirely

[NthPortal]

@dwijnand I am actually significantly concerned that this will lead to backsliding in unrelated areas. Can we instead change library (and the other modules) to have "-Wconf:msg=match may not be exhaustive:i"? (or w if that overrides -Werror, cc @som-snytt)

[dwijnand]

Yep, didn't think of that, thought we were blocked on re-STARR'ing and having access to the opt-outs. 👍

[som-snytt]

You two are talking way over my head. It should be called WAP for Warning Aware Programming.

Have you considered just using -nowarn? The question is whether to use -nowarn during development because warnings are noisy and annoying, or during PR validation because warnings are only for guiding development and they are too noisy and annoying for validation?