[clang] [clang-tools-extra] [Clang] Make the SizeType, SignedSizeType and PtrdiffType be named sugar types instead of built-in types (PR #143653)

[Matheus Izvekov via cfe-commits]

https://github.com/mizvekov commented:

Thanks, this looks good!

It would be awesome to use these in existing cases which we handle with a builtin typedef, as this would be helpful to validate the design, but this is not required and doesn't need to be part of this PR.

https://github.com/llvm/llvm-project/pull/143653