Perf: make SDoc monad one-shot (#18202)

With validate-x86_64-linux-deb9-hadrian:
   T1969  -3.4% (threshold: +/-1%)
   T3294  -3.3% (threshold: +/-1%)
   T12707 -1.4% (threshold: +/-1%)

Additionally with validate-x86_64-linux-deb9-unreg-hadrian:
   T4801  -2.4% (threshold: +/-2%)
   T13035 -1.4% (threshold: +/-1%)
   T13379 -2.4% (threshold: +/-2%)
   ManyAlternatives -2.5% (threshold: +/-2%)
   ManyConstructors -3.0% (threshold: +/-2%)

Metric Decrease:
    T12707
    T1969
    T3294
    ManyAlternatives
    ManyConstructors
    T13035
    T13379
    T4801

compiler/GHC/Utils/Outputable.hs

 {-# LANGUAGE LambdaCase #-}
+{-# LANGUAGE PatternSynonyms #-}
 
 {-
 (c) The University of Glasgow 2006-2012
@@ -121,6 +122,7 @@ import qualified Data.List.NonEmpty as NEL
 import GHC.Fingerprint
 import GHC.Show         ( showMultiLineString )
 import GHC.Utils.Exception
+import GHC.Exts (oneShot)
 
 {-
 ************************************************************************
@@ -304,7 +306,17 @@ code (either C or assembly), or generating interface files.
 -- To display an 'SDoc', use 'printSDoc', 'printSDocLn', 'bufLeftRenderSDoc',
 -- or 'renderWithContext'.  Avoid calling 'runSDoc' directly as it breaks the
 -- abstraction layer.
-newtype SDoc = SDoc { runSDoc :: SDocContext -> Doc }
+newtype SDoc = SDoc' (SDocContext -> Doc)
+
+-- See Note [The one-shot state monad trick] in GHC.Utils.Monad
+{-# COMPLETE SDoc #-}
+pattern SDoc :: (SDocContext -> Doc) -> SDoc
+pattern SDoc m <- SDoc' m
+  where
+    SDoc m = SDoc' (oneShot m)
+
+runSDoc :: SDoc -> (SDocContext -> Doc)
+runSDoc (SDoc m) = m
 
 data SDocContext = SDC
   { sdocStyle                       :: !PprStyle