Simplify way we tag unreachable code with only ISC_UNREACHABLE()

Simplify way we tag unreachable code with only ISC_UNREACHABLE()

Previously, the unreachable code paths would have to be tagged with:

    INSIST(0);
    ISC_UNREACHABLE();

There was also older parts of the code that used comment annotation:

    /* NOTREACHED */

Unify the handling of unreachable code paths to just use:

    UNREACHABLE();

The UNREACHABLE() macro now asserts when reached and also uses
__builtin_unreachable(); when such builtin is available in the compiler.

(cherry picked from commit 584f0d7a7e89f4681b78dee157f7a2807812023a)