Allow WARNING pragmas to be controlled with custom categories

Closes #17209. This implements GHC Proposal 541, allowing a WARNING
pragma to be annotated with a category like so:

    {-# WARNING in "x-partial" head "This function is undefined on empty lists." #-}

The user can then enable, disable and set the severity of such warnings
using command-line flags `-Wx-partial`, `-Werror=x-partial` and so on.  There
is a new warning group `-Wextended-warnings` containing all these warnings.
Warnings without a category are treated as if the category was `deprecations`,
and are (still) controlled by the flags `-Wdeprecations`
and `-Wwarnings-deprecations`.

Updates Haddock submodule.

1 files changed, 0 insertions, 0 deletions

diff --git a/utils/haddock b/utils/haddock
-Subproject 1f22a95c1db942fce2623b9daa26f66d193a4e7
+Subproject 6f1b9093395f4b12298b8b785b855a637206f5f