locking: teach Clang's -Wthread-safety to cope with our scoped lock macros

This allows us to use function/variable/class attributes to specify locking
requisites, allowing problems to be detected during static analysis.

This works perfectly with newer Clang versions (tested with 3.3-3.7). For older
versions (tested 3.2), it compiles fine but spews lots of false-positives.