EPA: remove duplicated comments when using '-haddock'
When the -haddock
GHC flag is set, the haddock comments are interspersed into the AST.
But they also appear with the "normal" comments.
At the moment exact printing ignores the ones interspersed in the tree (actually panics when they are hit),
and (badly) prints the structured comments.
Adjust the lexer to not emit the haddock EPA comments when the flag is set, and use the ones in the AST for exact printing.