Protect base/numerics against change in intmax_t's underlying type

Adds a static_assert to catch if this ever changes, because that will
require changing some type promotion logic and other assumptions.

TBR=tsepez@chromium.org

Review-Url: https://codereview.chromium.org/2506093004
Cr-Commit-Position: refs/heads/master@{#433085}
1 file changed