We previously missed out on turning ConsString objects into ThinStrings
due to the flattening step in StringTable::LookupString.

  // Example input string:
  string = cons("A", cons"B", "C"))
  // After flatten:
  string == cons("ABC", "")
  // OLD: After internalising and thinning:
  string == cons(#"ABC", "")

The new behaviour yields a thin string after internalising:

  // NEW: After internalising and thinning:
  string == thin(#"ABC")

Change-Id: I99bbebd52fa02ecfeb2289cb85b69f3e01175cdd
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3667080
Commit-Queue: Camillo Bruni <cbruni@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/main@{#81004}