Xcode 4.2 fixes

[MikeMcQuaid]

Use Clang on fails_with_llvm and bump the highest seen LLVM version to that from Xcode 4.2.

Again I'm wondering if we should set Clang to the default compiler on Xcode 4.2 or keep using the /usr/bin/{cc,c++} symlinks to decide.

[jacknagel]

I think this is fine, but we should keep trying to respect /usr/bin/cc when possible.

It's hard to say "this version of OS X with this version of XCode should use ____" because there are corner cases. For example, I'm on Snow Leopard, using XCode 4.2, and I have both gcc 4.0 and 4.2 left over from previous XCode installs. And some Lion users who upgraded XCode still have gcc 4.2 around.

I'm trying to address this in my work to make fails_with more generic by avoiding checking the OS X/XCode versions unless absolutely necessary and relying almost entirely on compiler build numbers instead; I'll try to have something reviewable this week.

[MikeMcQuaid]

Ok. Will merge this as-is. Thanks.

[jacknagel]

And I agree that in general, we should be encouraging clang for users with the most up-to-date tools.

[MikeMcQuaid]

Cool, glad we're agreed. Will leave it as-is until you've got something to look at. Ping me then.