cmake: Add optional clang-tidy integration

clang-tidy performs static analysis over the compilation unit,
producing more comprehensive compile warnings than are normally
generated by the compiler. For example, it will trace branches in a
function to find the exact conditions in which a null dereference
can occur.

Note that this produces an absurd amount of warnings for our existing
C code, and significantly slows compile times. It is pretty effective
at its job though. I think it would be good to turn on by default for
upcoming C++ code.