[clang] [Clang][CodeGen] Use ptrtoaddr for pointer diff (PR #210729)

[Alexis Engelke via cfe-commits]

aengelke wrote:

I think this should be ready now. On CTMark, the remaining [size changes](https://llvm-compile-time-tracker.com/compare.php?from=7bc2770064d3afbb4ce49c159d99c69a9e4b5c5b&to=037eaf74d30cbec0cafba1fd74fdb0ce0d53cb48&stat=size-text) are small.

https://github.com/llvm/llvm-project/pull/210729