Speed up class_getImageName

[tbkka]

When building for Apple OS, dyld_image_path_containing_address
is much, much faster than dladdr.

Resolves rdar://42137685
Resolves SR-14759

[tbkka]

@swift-ci Please test

[allevato]

Thanks!

Out of curiosity, do the CI presets use a configuration that builds the core libraries for in-the-OS distribution, so that mach-o/dyld_priv.h is available and the new code path gets utilized?

[mikeash]

Alas no, CI builds against the public SDK and so it won't see dyld_priv.h. Might be worth bringing our own declaration so that we can use the same code path in CI for more accurate testing.

[tbkka]

I thought we specifically wanted to avoid any non-public calls unless building for in-the-OS distribution?

[mikeash]

We have quite a few others already. The main thing is just to make sure we can support building with older SDKs (not sure how far back we actually intend to support, but often we're supporting the previous year when calls show up) which is why we look up a lot of stuff dynamically. This call is pretty old so that's not a concern.

[mikeash]

I should say, building with older SDKs and running on slightly older OSes. We still support development on macOS 11 currently, so the runtimes built by developers still need to work on macOS 11. But still not an issue here.