Implement basic uniform warning set tower

This implements/completes the current basic warning sets to provide the
following tower of warning sets (i.e. each line subsumes the warnings
from the sets listed below):

- `-Weverything`
- `-Wall`
- `-Wextra` (alias of `-W`)
- `-Wdefault`

So for each of flags there's also a complement `-Wno-...` flag, which
subtracts the given set from the current enabled-warnings state.

Thus, we can now easily perform simple set subtraction operations, as
warning flags are evaluated from left-to-right on the command line.

So e.g.

- `-Weverything -Wno-all -Wno-compat` enables *all* warnings not enabled
  by `-Wall` and `-Wcompat`.

- `-Wextra -Wno-default` only warnings that `-Wextra` provides
  beyond the default warnings.

Reviewers: quchen, austin, bgamari

Reviewed By: bgamari

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D1850

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