EPA: Use less cryptic names for EpaAnchor constructors

Arising from #19579 and !2418 (comment 338142)

At the moment EpaAnchor is defined with short, cryptic constructors as

data EpaAnchor = AR RealSrcSpan
               | AD DeltaPos
               deriving (Data,Show,Eq,Ord)

Change it to

data EpaAnchor = AnchorReal RealSrcSpan
               | AnchorDelta DeltaPos
               deriving (Data,Show,Eq,Ord)