Re CLC issue 292 Warn GHC.Internal.List.{init,last} are partial

See:

• https://github.com/haskell/core-libraries-committee/issues/292

The Haddock documentation for each of init and last of base-4.20.0.1 includes:

WARNING: This function is partial. Consider using unsnoc instead.

This MR seeks to:

• add {-# WARNING in "x-partial" ... #-} to GHC.Internal.List.{init,last} with a similar message, namely: This is a partial function, it throws an error on empty lists. Use unsnoc instead. Consider refactoring to use "Data.List.NonEmpty".;
• add similar warnings to Haddock documentation for GHC.Internal.List.{head,tail} (as {-# WARNING #-} does not feature in Haddock documentation on Hackage for base >= 4.20.0.0 because the documentation is built with an old version of Haddock);
• add {-# OPTIONS_GHC -Wno-x-partial #-} where init and/or last are used in GHC's repository. {-# OPTIONS_GHC -Wno-unrecognized-warning-flags -Wno-x-partial #-} is used for modules that are currently built with GHC versions before GHC 9.8.1; and
• in Hadrian module Settings.Warnings, applies -Wno-unrecognized-warning-flags -Wno-x-partial to the Cabal, filepath, hsc2hs, hpc, parsec, text and time packages (outside GHC's repository).

(testsuite\tests\hiefile\should_run\T20341.hs is a little tricky to update, because if T20341.hie is not as expected, there seems to be little to help you work out what might be wrong. I bumped all the line counts by 3.)