Investigate old comment at the top of SrcLoc

There is a comment at the top of SrcLoc which says

{-# OPTIONS_GHC -fno-omit-interface-pragmas #-}                                 
  -- Workaround for Trac #5252 crashes the bootstrap compiler without -O       
  -- When the earliest compiler we want to boostrap with is                    
  -- GHC 7.2, we can make RealSrcLoc properly abstract   

This is a well-defined small task so I will mark it for newcomers.

1. Try to work out what the pragma is meant to do by reading #5252 (closed)
2. Work out whether we can now make RealSrcLoc "properly abstract" (if it is not already)
3. Remove or update this comment and pragma.
4. Pray it doesn't break the whole compiler.