Force some clang Werrors that *will* break things.

Clang is really aggressive at optimizing a handful of cases (read:
clang will ruin your day some if you write bad code). Fortunately, it
also emits a warning when it's about to do this.

To prevent anyone from suffering from these optimizations, make these
warnings errors and make them impossible to disable.

Change-Id: I5e10bb0fc2ca23190017da716b3b84635577a0bd