Ppr: compute length of string literals at compile time (#19266)

Commit message:

SDoc string literals created for example with `text "xyz"` are converted
into `PtrString` (`Addr#` + size in bytes) with a rewrite rule to avoid
allocating a String.

Before this patch, the size in bytes was still computed at runtime. For
every literal, we obtained the following pseudo STG:

    x :: Addr#
    x = "xzy"#

    s :: PtrString
    s = \u [] case ffi:strlen [x realWorld#] of
            (# _, sz #) -> PtrString [x sz]

But since GHC 9.0, we can use `cstringLength#` instead to get:

    x :: Addr#
    x = "xzy"#

    s :: PtrString
    s = PtrString! [x 3#]

Literals become statically known constructor applications. Allocations
seem to decrease a little in perf tests (between -0.1% and -0.7% on CI).

compiler/GHC/Data/FastString.hs

@@ -6,6 +6,7 @@
 {-# LANGUAGE GeneralizedNewtypeDeriving #-}
 {-# LANGUAGE MagicHash #-}
 {-# LANGUAGE UnboxedTuples #-}
+{-# LANGUAGE UnliftedFFITypes #-}
 
 {-# OPTIONS_GHC -O2 -funbox-strict-fields #-}
 -- We always optimise this, otherwise performance of a non-optimised
@@ -425,6 +426,7 @@ lower-level `sharedCAF` mechanism that relies on Globals.c.
 -}
 
 mkFastString# :: Addr# -> FastString
+{-# INLINE mkFastString# #-}
 mkFastString# a# = mkFastStringBytes ptr (ptrStrLength ptr)
   where ptr = Ptr a#
 
@@ -653,6 +655,7 @@ data PtrString = PtrString !(Ptr Word8) !Int
 
 -- | Wrap an unboxed address into a 'PtrString'.
 mkPtrString# :: Addr# -> PtrString
+{-# INLINE mkPtrString# #-}
 mkPtrString# a# = PtrString (Ptr a#) (ptrStrLength (Ptr a#))
 
 -- | Encode a 'String' into a newly allocated 'PtrString' using Latin-1
@@ -688,8 +691,14 @@ lengthPS (PtrString _ n) = n
 -- -----------------------------------------------------------------------------
 -- under the carpet
 
+#if !MIN_VERSION_GLASGOW_HASKELL(9,0,0,0)
 foreign import ccall unsafe "strlen"
-  ptrStrLength :: Ptr Word8 -> Int
+  cstringLength# :: Addr# -> Int#
+#endif
+
+ptrStrLength :: Ptr Word8 -> Int
+{-# INLINE ptrStrLength #-}
+ptrStrLength (Ptr a) = I# (cstringLength# a)
 
 {-# NOINLINE sLit #-}
 sLit :: String -> PtrString