Haddock comments on infix constructors (#24221)

Rewrite the `HasHaddock` instance for `ConDecl GhcPs` to account for
infix constructors.

This change fixes a Haddock regression (introduced in 19e80b9a)
that affected leading comments on infix data constructor declarations:

	-- | Docs for infix constructor
	| Int :* Bool

The comment should be associated with the data constructor (:*), not
with its left-hand side Int.