EPA: EpaDelta for comment has no comments

EpaLocation is used to position things. It has two constructors,
EpaSpan holding a SrcSpan, and EpaDelta with a delta position and a
possible list of comments.  The comment list is needed because the
location in EpaDelta has no absolute information to decide which
comments should be emitted before them when printing.

But it is also used for specifying the position of a comment.  To
prevent the absurdity of a comment position having a list of comments
in it, we make EpaLocation parameterisable, using comments for the
normal case and a constant for within comments.

Updates haddock submodule.

aarch64-darwin
Metric Decrease:
    MultiLayerModulesTH_OneShot