base: add COMPLETE pragma to BufferCodec PatSyn

This implements CLC proposal #178, rectifying an oversight in the
implementation of CLC proposal #134 which could lead to spurious
pattern match warnings.

https://github.com/haskell/core-libraries-committee/issues/178
https://github.com/haskell/core-libraries-committee/issues/134
(cherry picked from commit 22565506)

libraries/base/GHC/IO/Encoding/Types.hs

@@ -145,6 +145,7 @@ data CodingProgress = InputUnderflow  -- ^ Stopped because the input contains in
                              , Show -- ^ @since 4.4.0.0
                              )
 
+{-# COMPLETE BufferCodec #-}
 pattern BufferCodec :: CodeBuffer from to
                     -> (Buffer from -> Buffer to -> IO (Buffer from, Buffer to))
                     -> IO ()