shims: make xcrun look at superenv bin first

[Bo98]

• [x] Have you followed the guidelines in our Contributing document?
• [x] Have you checked to ensure there aren't other open Pull Requests for the same change?
• [x] Have you added an explanation of what your changes do and why you'd like us to include them?
• [ ] Have you written new tests for your changes? Here's an example.
• [x] Have you successfully run brew style with your changes locally?
• [ ] Have you successfully run brew typecheck with your changes locally?
• [ ] Have you successfully run brew tests with your changes locally?

---

This closes a superenv bypass where build systems using xcrun clang or xcrun --find clang returned the path directly to Xcode's clang. This meant formulae like qt@5 has never used our superenv clang.

This does require a bit of arg parsing in our xcrun shim which is unfortunate (need to watch for changes to system xcrun - though the tool has been stable for a while), but necessary to replicate the behaviour of system xcrun.

[BrewTestBot]

Review period will end on 2021-12-03 at 16:34:50 UTC.

[BrewTestBot]

Review period ended.

[carlocab]

Related to Qt and superenv: https://code.qt.io/cgit/pyside/pyside-setup.git/tree/sources/shiboken6/ApiExtractor/clangparser/compilersupport.cpp#n118

[github-actions[bot]]

This pull request has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs.

[carlocab]

This fixes a bug that confuses Ruby's build into trying to install its headers into the SDK. See Homebrew/homebrew-core#92024.

[github-actions[bot]]

This pull request has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs.