Introduce IsPass; refactor wrappers.

There are two main payloads of this patch:

1. This introduces IsPass, which allows e.g. printing
   code to ask what pass it is running in (Renamed vs
   Typechecked) and thus print extension fields. See
   Note [IsPass] in Hs.Extension

2. This moves the HsWrap constructor into an extension
   field, where it rightly belongs. This is done for
   HsExpr and HsCmd, but not for HsPat, which is left
   as an exercise for the reader.

There is also some refactoring around SyntaxExprs, but this
is really just incidental.

This patch subsumes !1721 (sorry @chreekat).

Along the way, there is a bit of refactoring in GHC.Hs.Extension,
including the removal of NameOrRdrName in favor of NoGhcTc.
This meant that we had no real need for GHC.Hs.PlaceHolder, so
I got rid of it.

Updates haddock submodule.

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Metric Decrease:
    haddock.compiler
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