[RFC] func_attr: clang/icc support NONNULL_* attr

Both clang and (apparently) icc support the nonnull attribute. So I'm not
sure why it wasn't enabled in the first place.

- clang: http://clang-analyzer.llvm.org/annotations.html#attr_nonnull
- icc: https://software.intel.com/en-us/forums/topic/280290 (not conclusive
  evidence, but the best I could find, lacking docs)