Use Xcode's clang, not Chromium's

Chromium has its own checked-in version of clang. Using this is cool: it gives us a nice modern clang and also supports some Chromium-specific clang plugins.

But using it is also problematic: since embedding applications probably won't be using Chromium's clang, there can be errors and bugs due to differences between toolchain versions. For example, Chromium's clang currently includes http://llvm.org/viewvc/llvm-project?view=revision&revision=184250 while Xcode's clang does not, leading to linker errors due to different name mangling.

So now we build with Xcode's clang instead of Chromium's. This means we can't use Chromium's clang plugins, but those seem to mostly be for catching style errors etc.