Don't forget about a builtin if we're about to redeclare it and we couldn't
create an implicit declaration of it (because some type it depends on is
unavailable). This had the effect of causing us to not implicitly give it the
right attributes. It turns out that glibc's __sigsetjmp is declared before
sigjmp_buf is declared, and this resulted in us not implicitly giving it
__attribute__((returns_twice)), which in turn resulted in miscompiles in any C
code calling glibc's sigsetjmp.

(See also the vaguely-related sourceware.org/PR4662.)