[clang] [clang-tools-extra] [libcxx] [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 approved this pull request.

I think this looks good, sans a single nit, thanks!

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