[native_toolchain_c] Fix clang compiler search + tools when using swiftly

[nikeokoronkwo]

Fixes #2792

Swiftly proxies commands to some of the tools that ship with Swift distributions, including: clang, ld.lld, and other tools. Therefore, such tools are symlinks to swiftly, and running swiftly directly would therefore not work when bundling assets.

This PR adds support for using package:native_toolchain_c on macOS with Swiftly installed, defaulting to the user's default installation of ar and ld where applicable.

I assume that in realistic use-cases, users would want to use specific toolchains and therefore, would want to use tools together rather than distinct instances of certain tools (i.e. prefer ld bundled with the currently resolved clang than a separate use of ld), and most users would prefer using the Clang distribution they already have in their system rather than that bundled with Swiftly, so I'm considering a separate PR for using a different clang toolchain than that Swiftly has. It would require some additions to current tool resolving and behavior, so I was considering it on a separate PR than this one.

[dcharkes]

Thanks @nikeokoronkwo !

It would require some additions to current tool resolving and behavior, so I was considering it on a separate PR than this one.

👍

If it is possible to have swiftly on the CI here in GitHub, you could consider adding an extra github workflow that covers it. We also run the CI an extra time in .github/workflows/native_toolchain_c.yaml to cover a different toolchain than the default ones.

And also, please add some tests that tools don't resolve at all on Linux/Windows.

(And version + changelog entry so I can hit publish after merging 😄)

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