Fixes bug 22644; bugfix on 0.2.7.1-alpha and 0.2.0.1-alpha
respectively.

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.

Wow, it sure seems like some compilers can't implement isnan() and
friends in a way that pleases themselves!

Fixes bug 22915. Bug trigged by 0.2.8.1-alpha and later; caused by
clang 4.

(cherry picked from commit c91a57ccf90308c6728184b43519f96b61acb95d)

(cherry picked from commit 1bb00fb812c0df7a574ed62e9f53b0e8192c7d04)

(cherry picked from commit 8f8689f70235dc19cbc5092ea148af5772a9cdc3)