Update Xcode versions for apple-clang builds

[rothmichaels]

Since mp-units requires at least Xcode 15 we should have coverage on the latest compilers for operating system versions for macOS 13 (Ventura) and macOS 14 (Sequoia) as those are the operating systems with necessary compilers that our users would be using.

Now build with Xcode 15.2 instead of 15.0.1 which last version supported on macOS 13.

Add a build for Xcode 15.4 and Xcode 16.1 for testing macOS 14 builds. Since Xcode 16.1 is new most of our clients using macOS 14 would might more likely be using Xcode 15.4.

There might not be much C++ changes between Xcode 15.2 and Xcode 15.4 so perhaps we should drop 15.4 if adding three apple-clang builds is not worth the resources.

[rothmichaels]

@mpusz Let me know what you think and if we should drop 15.4 from the matrix — not sure what resources are costing us for adding to the test matrix.

[mpusz]

I do not know what are the differences between 15.2 and 15.4. Are there any significant apple-clang version changes?

Also, could you please check if any of the new versions support additional features from https://mpusz.github.io/mp-units/latest/getting_started/cpp_compiler_support?

[rothmichaels]

I do not know what are the differences between 15.2 and 15.4. Are there any significant apple-clang version changes?

I don't think a lot (the performance bugs exist in both). There were some apple-clang updates in 15.3 that you can see here https://developer.apple.com/documentation/xcode-release-notes/xcode-15_3-release-notes

I think all of Xcode 15 is based on some fork of clang/llvm 16.

Also, could you please check if any of the new versions support additional features from https://mpusz.github.io/mp-units/latest/getting_started/cpp_compiler_support?

I think maybe there is some std::format support? I can try that out tomorrow.

[mpusz]

According to my knowledge from mainstream clang versions, std::format support was added in clang-17. If 15.4 is still based on clang-16 then we probably do not have to test it.

For sure we need to build on the latest macOS 13 version (15.2) to not drop our users. And of course we need to test Xcode 16 from macOS 14.

[mpusz]

Thanks!