[mono][net7] Cherrypick fixes to build runtime with Xcode 15

[lambdageek]

Our official builds use an older Xcode for creating .NET 7 packages, but if we ever upgrade the build machines here's what we will need to cherrypick to get net7 building with Xcode 15:

• [iOS] Add temp cindex attribute to build on xcode 15 #88261
  • Not sure if we need the App Builder changes; for sure we need the cindex.py changes
• [mono] Update offsets-tool python clang binding so it works with newer clang #86256
• https://github.com/dotnet/runtime/pull/87190/files#diff-1b1edde0c18ae50547eda0255286553a9cdc5d8d2e381b6cca7db559e3bd6215 - just the offsets-tool.py changes from that PR

[lambdageek]

/cc @steveisok @directhex @akoeplinger

[lambdageek]

CoreCLR macOS builds probably need changes too, although I don't know what those might entail

[steveisok]

@akoeplinger please take care of this.

[akoeplinger]

PR is in #93543

Not sure if we need the App Builder changes; for sure we need the cindex.py changes

I decided not to backport the AppBuilder changes since they're only needed for tests and we probably can live without them for now.

CoreCLR macOS builds probably need changes too, although I don't know what those might entail

I built CoreCLR with xcode 15 and it built fine without changes.