extend/ENV/std: fix llvm_clang usage in stdenv

[cho-m]

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same change?
• 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.
• 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?

---

Currently some tests require ENV.clang or ENV.method(DevelopmentTools.default_compiler).call to bypass the llvm_clang compiler selector can pick since standard environment doesn't have this in PATH.

[cho-m]

I may want to add the LLVM header shim to standard environment before merge to work around same issue as superenv. So:

• Move shims/mac/super/include --> shims/mac/shared/include
• Maybe use CPLUS_INCLUDE_PATH to inject it (not sure if other flags will be used automatically).

[MikeMcQuaid]

@cho-m both sound sensible to me