Warn about badly staged types

Consider an example:

% ghci-9.6.2                              
GHCi, version 9.6.2: https://www.haskell.org/ghc/  :? for help
Prelude> :set -XTemplateHaskell
Prelude> :m +Data.Proxy Language.Haskell.TH
Prelude Data.Proxy Language.Haskell.TH> let tardy :: forall a. Proxy a -> IO Type; tardy _ = [t| a |]
Prelude Data.Proxy Language.Haskell.TH> tardy (Proxy @Int)
VarT a_6989586621679012786

That's most likely not what user expects. I'd expect that to print the same as

Prelude Data.Proxy Language.Haskell.TH> [t| Int |]
ConT GHC.Types.Int

But someone might be relying on this, so I propose to first introduce a warning and see if those stage errors exist (it could be warning with default error level, but AFAIK we don't yet have precedent for such warnings).