Various error messages have inaccurate source locations

Some error messages lack source location information, or have inaccurate locations. Here are the ones we know about:

• Should point to the import decl:

   ShowFunctions.hs:1:0
     Warning: Module `Text.Show.Functions' is imported, but nothing from it is used

• Should point to the instance header:

   mod41.hs:3:0:
       Illegal instance declaration for `Eq (Either a a)'
         (The instance type must be of form (T a b c)

• Should point to 'deriving *Eq*', not the tycon:

   tcfail046.hs:9:8:
     No instance for `Eq (Pid -> Time -> Message a -> (MessList a, Continuation a
))'
     When deriving the `Eq' instance for type `Continuation'

• check_tau_type doesn't have location info?

   tcfail100.hs:7:0:
       Type synonym `A' should have 1 argument, but has been given 0
       In the type synonym declaration for `B'

• Location in LHsModule from the parser should really span the whole file, rather than a point span at (1,0).
• read016: should be the lhs only?
• tcfail044: should be the instance head only.