Remove the MonadFail P instance

There were two issues with this instance:

* its existence meant that a pattern match failure in the P monad would
  produce a user-visible parse error, but the error message would not be
  helpful to the user

* due to the MFP migration strategy, we had to use CPP in Lexer.x,
  and that created issues for #17750

Updates haddock submodule.

1 files changed, 0 insertions, 0 deletions

diff --git a/utils/haddock b/utils/haddock
-Subproject d838d08f0ac0173dc704d51191b1c1976964b6f
+Subproject 40591606251693956d9729ab3a15c7244d7fc2a