LINE and COLUMN pragmas

I recently discovered the `UsePosPragsBit` in the lexer, which can allow emitting the LINE and COLUMN pragmas as tokens instead of processing them directly in the Lexer, updating the position. When exact printing, this behaviour is not desirable. So I am putting together an MR to parse them if they appear, so they do not cause a parser crash, and they can then be exact printed. Question: Where should these pragmas appear in HsDecl Options are - TTG, XXHsDecl - A new HsDecl constructor, with all its concomitant matching everywhere - tuck it in elsewhere, e.g. in `AnnDecl` (or `XXAnnDecl`). Alan