Accept strings as values of attributes

This patch is the first step to solve #11886. It allows quoted strings as values of attributes and uses this syntax for `lang` items, while keeping the old syntax `lang = "foo"` valid. I've created another branch, which changes all `lang = "foo"` attributes to `lang("foo")`, because this only works after this PR got merged and is in the current snapshot (It seems like #[cfg(stage0)] does not work any more). The relevant commit can be found in my fork: https://github.com/fhahn/rust/commit/41527f72909c85c8b77242f444ed9bcdcdbedd5b

With this patch, quoted strings are accepted as values of all attributes, not only for `lang` attributes, but I'm not sure if this is intended.

Also, I want to add a few tests, because it seems like there are not tests for `lang` at the moment.