os/mac/xcode: add fast path for Xcode version detection

[Bo98]

At best, MacOS::Xcode.detect_version would take ~0.12s - and likely even slower in CI environments.

This PR improves this by about 10x (100x if MacOS.active_developer_dir has already been called somewhere), by finding and reading version.plist rather than a slow shell-out to xcodebuild.

On CLT-only installs, we skip xcodebuild execution entirely and go straight to clang version guessing. It's debatable whether MacOS::Xcode.version should really be returning anything for CLT-only installs, but I understand why it's been chosen to behave that way.

[apainintheneck]

Code looks good. I was able to repeat the results and the performance improvement locally.

This is a just micro-optimization, right? It seems like the result of Xcode.detect_version always gets memoized.

Nit: The Xcode.detect_version method is getting a bit large. It might make sense to break it up a little more.

[Bo98]

This is a just micro-optimization, right? It seems like the result of Xcode.detect_version always gets memoized.

Sort of. Micro in the sense you probably won't notice it on most machines but large enough that you will see clearly it in a brew prof output. It will trigger in the parent brew process and in build.rb subprocess so any effects there are doubled (build.rb/postinstall.rb startup is something in particular I'd like to see improved), so a quarter of a second is a fair bit. You can multiply the effect further by considering brew test-bot runs from cold states for each command run, but that's minor compared to the overall run.

It's one of those things I saw in the brew prof graph, looked at the code and said "why are we doing that way?". We could probably delete the xcodebuild code entirely to be honest - this will also solve your large method concern.

[MikeMcQuaid]

Nice work @Bo98!