Add BufSpan to EpaLocation (#22319, #22558)

1st step of #22558

The key part of this patch is the change to mkTokenLocation:

- mkTokenLocation (RealSrcSpan r _)  = TokenLoc (EpaSpan r)
+ mkTokenLocation (RealSrcSpan r mb) = TokenLoc (EpaSpan r mb)

mkTokenLocation used to discard the BufSpan, but now it is saved and can be retrieved from LHsToken or LHsUniToken.

This is made possible by the following change to EpaLocation:

- data EpaLocation = EpaSpan !RealSrcSpan
+ data EpaLocation = EpaSpan !RealSrcSpan !(Strict.Maybe BufSpan)
                   | ...

The end goal is to make use of the BufSpan in Parser/PostProcess/Haddock.