- `-fsanitize=undefined` encompasses multiple checks we were specifying
- "detect_leaks=1" for `__asan_default_options` checks for memory leaks
(except for with macOS clang++, which does not support LSan)
- `-fsanitize=float-divide-by-zero` is an extra UBSan check
(and reveals a UB bug to fix with fixed-point `DIV` and `LOG`)
Should not be followed for C files, to avoid mucking `blame`s,
but will be applied during the gradual C++ rollout, since that
will likely touch most lines in the codebase anyway.
