Don't include BufPos in interface files
Ticket #22162 (closed) pointed out that the build directory was leaking into the ABI hash of a module because the BufPos depended on the location of the build tree.
BufPos is only used in GHC.Parser.PostProcess.Haddock, and the information doesn't need to be propagated outside the context of a module.
Fixes #22162 (closed)