Fixes Trac #925

A new comment in TcRnDriver in tcRnStmt reads thus: 

At one stage I removed any shadowed bindings from the type_env;
they are inaccessible but might, I suppose, cause a space leak if we leave them there.
However, with Template Haskell they aren't necessarily inaccessible.  Consider this
GHCi session
	 Prelude> let f n = n * 2 :: Int
	 Prelude> fName <- runQ [| f |]
	 Prelude> $(return $ AppE fName (LitE (IntegerL 7)))
	 14
	 Prelude> let f n = n * 3 :: Int
	 Prelude> $(return $ AppE fName (LitE (IntegerL 7)))
In the last line we use 'fName', which resolves to the *first* 'f'
in scope. If we delete it from the type env, GHCi crashes because
it doesn't expect that.