Untyped TemplateHaskell as a bedrock for macros (Was: Bug in renaming of TemplateHaskell quotes)

(Extracted from #24713 (comment 563662).) The following program

{-# LANGUAGE TemplateHaskell #-}

main = do
  print $ $(let x = 10::Int in [| let x = 1 in $[| x |] |])
  print $ $(let x = 10::Int in let q = [| x |] in [| let x = 1 in $q |])

should print 10 twice in a usable procedural macro system such as Leans, but in current TH it prints 1\n10.

That is due to a bug feature in the renamer, which converts the latter into the former, but then analyses [| x |] in the environment of its splice site $q.

This is easily fixed by introducing a TcLvlEnv field to RnPendingUntyped that carries the renaming environment before entering the outer quote.