TTG follow-up: Template Haskell related

These are TODOs regarding TTG to follow !4782 (closed):

Quoting @simonpj:

I see that UntypedSpliceFlavour is used only in the client specific GHC.Hs.Expr; but it is defined in the client-independent L.H.S.Expr. Let's move it next to PendingRnSplice.

In L.H.S.Expr I see

 | HsQuasiQuote        -- See Note [Quasi-quote overview] in GHC.Tc.Gen.Splice

But that Note doesn't exist. Can we find it from history?

In L.H.S.Expr I see

data HsSplice id
 = HsTypedSplice       --  $$z  or $$(f 4)
      (XTypedSplice id)
      SpliceDecoration -- Whether $$( ) variant found, for pretty printing
      (IdP id)         -- A unique name to identify this splice point
      (LHsExpr id)     -- See Note [Pending Splices]

The (IdP id) part is very GHC-specific. It should be in XTypedSplice.

Formally in #16830 (closed)

• HsSpliced should be moved to XSplice. See the TODO there from @alanz.

From @Ericson2314

• Inline HsSplice altogether!

Just as we now have Hs{Untyped,Typed}Bracket, untyped and typed splices, and quasiquotes, are all distinct syntax, and come from morally distinct extensions. They should have separate top-level AST nodes too, for that reason, and for consistency's sake!

To close this issue we should

• Move UntypedSpliceFlavour to client specific GHC.Hs.Expr next to PendingRnSplice
• Find Note [Quase-quote overview] in GHC.Tc.Gen.Splice that no longer exists from history
• Refactor IdP id GHC-specific part into XTypedSplice from the constructor of HsSplice in L.H.S.Expr