revert rust's clang iOS6 buildsystem hotpatch

[erickt]

According to the llvm developers, the clang bug that broke compiling rust on snow leopard has been fixed:

http://llvm.org/bugs/show_bug.cgi?id=14013

So we should revert our fix so we're on the mainline clang version.

[pnkfelix]

This seems like a nice simple task to do right after the 0.6 release.

[pnkfelix]

Not critical for 0.6; de-milestoning

[graydon]

clang has been disabled for the time being; if this re-emerges when we re-enable it in the future, we may re-discover it, but that is too hypothetical to keep a bug open for.