Install libc++ package on CI images of Ubuntu 24.04

[egorzhdan]

This adds a libc++-18-dev package to the list of packages installed on a CI image of Ubuntu 24.04.

Swift is now capable of compiling code that uses C++ interop with libc++ on Linux. Having the libc++ package installed provides us with CI coverage of the feature.

This is similar to #402.

[finagolfin]

This will probably fail because of swiftlang/swift#76325, as I just tried building those new libc++ tests you added natively on Android with the libc++ 18 from Android NDK 27b and both failed.

You may have to stick with libc++ 16 until that problem is fixed.

[egorzhdan]

I haven't been able to reproduce the libc++ issue you're seeing. I tried both macOS and Linux. I suspect this is something Android-specific.

[finagolfin]

OK, good that it works for you on linux/mac, but I don't see how it could be Android-specific when it just uses the same libc++ and accompanying C++ module map pretty much, while the failing header is in libc++ itself.

I need to look into those C++ Interop tests that also fail on Android with this libc++ 17 re-org, so I will do some digging.

[egorzhdan]

@finagolfin we aren't building the compiler with libc++ on Linux, we're only testing it with libc++, so there isn't a risk of the failure you were seeing from this patch.

[finagolfin]

Sure, but my subsequent point in this thread stands that these new libc++ tests of yours in the compiler validation suite failed natively on Android with its libc++ 18, which isn't very different from the libc++ package on linux that you're adding here.

I'm not saying don't merge this, as it's certainly possible I'm seeing failures specific to Android somehow, just warning you that's what I saw on another non-CI platform and I'm trying to figure out what might be going wrong.

If you're confident this will work well for linux, don't worry about Android. I will look into those Android failures later and report back.