In the Linux kernel, the BUG() macro causes an instant panic.  Our
BUG() macro is different, however: it generates a nonfatal assertion
failure, and is usable as an expression.

Additionally, this patch tells util_bug.h to make all assertion
failures into fatal conditions when we're building with a static
analysis tool, so that the analysis tool can look for instances
where they're reachable.

Fixes bug 23030.