Fix for indexInt8OffAddr# returning sized type in next GHC

Here's how we fix this without CPP:

In old GHC:

  I8# :: Int# -> Int8
  indexInt8OffAddr# :: Addr# -> Int# -> Int#

In upcoming GHC 9.2:

  I8# :: Int8# -> Int8
  indexInt8OffAddr# :: Addr# -> Int# -> Int8#

So the "GLB" interface is:

  exists alpha.
  I8# :: alpha -> Int8
  indexInt8OffAddr# :: Addr# -> Int# -> alpha

We we write a program against that, eliminating the black-box `alpha`
with `I8#` and then converting to `Int`.

containers/src/Data/IntSet/Internal.hs

@@ -221,6 +221,7 @@ import Text.Read
 
 #if __GLASGOW_HASKELL__
 import qualified GHC.Exts
+import qualified GHC.Int
 #endif
 
 import qualified Data.Foldable as Foldable
@@ -1642,7 +1643,7 @@ highestBitSet x = WORD_SIZE_IN_BITS - 1 - countLeadingZeros x
 ----------------------------------------------------------------------}
 
 indexOfTheOnlyBit bitmask =
-  GHC.Exts.I# (lsbArray `GHC.Exts.indexInt8OffAddr#` unboxInt (intFromNat ((bitmask * magic) `shiftRL` offset)))
+  fromIntegral (GHC.Int.I8# (lsbArray `GHC.Exts.indexInt8OffAddr#` unboxInt (intFromNat ((bitmask * magic) `shiftRL` offset))))
   where unboxInt (GHC.Exts.I# i) = i
 #if WORD_SIZE_IN_BITS==32
         magic = 0x077CB531