Remove usages of assert(), add PRESUBMIT checks

assert() is controlled by NDEBUG, but that's confusing because we
actually control asserts via DAWN_ENABLE_ASSERTS. Make all asserts
consistently use DAWN_ASSERT and DAWN_ENABLE_ASSERTS.

DAWN_ASSERT is also better because it avoids unused variable warnings.
(The genesis of this change was discovering one [[maybe_unused]] that
was only necessary because of an assert().)

Adds both PRESUBMIT and Clang-Tidy rules to try to enforce this.
Unfortunately the NDEBUG check won't be totally warning-clean because
sometimes we actually want that and not DAWN_ENABLE_ASSERTS. In fact I
didn't find any place we used it wrong - there's only one place NDEBUG
is used; the other one that was incorrect was already fixed:
https://dawn.googlesource.com/dawn/+/098fd8d2225013faec7bfaa3bccd2022a19cb19f

Note there are some places where assert() is implicitly in scope for
some reason, but we didn't explicitly include <cassert>. These are
detected best-effort (if they're single-line) by PRESUBMIT. assert() is
a macro, not a function, so we can't detect these with
bugprone-unsafe-functions.CustomFunctions.

Bug: 501491694
Change-Id: I583edcedddc09c8f11d1315c6de76957e6f5d498
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/345796
Reviewed-by: dan sinclair <dsinclair@chromium.org>
Commit-Queue: Kai Ninomiya <kainino@chromium.org>