[StackProtector] attribute __stack_chk_fail as NoReturn

When GCC added support for stack smashing protections, it was defined
that:

> This hook returns a CALL_EXPR that alerts the runtime that the stack
> protect guard variable has been modified. This expression should
> involve a call to a noreturn function.
> The default version of this hook invokes a function called
> ‘__stack_chk_fail’, taking no arguments.

Do so as well for __stack_smash_handler for OpenBSD.

Every libc implementation I could find has __stack_chk_fail marked
noreturn, or the implementation calls abort, exit, or panic (which
themselves are noreturn).

Glibc: https://sourceware.org/git/?p=glibc.git;a=blob;f=debug/stack_chk_fail.c
Musl: https://git.musl-libc.org/cgit/musl/tree/src/env/__stack_chk_fail.c
Bionic: https://android.googlesource.com/platform/bionic/+/refs/heads/master/libc/bionic/__stack_chk_fail.cpp
FreeBSD: https://cgit.freebsd.org/src/tree/lib/libc/secure/stack_protector.c
OpenBSD: https://github.com/openbsd/src/blob/master/lib/libc/sys/stack_protector.c
NetBSD: https://github.com/NetBSD/src/blob/trunk/lib/libc/misc/stack_protector.c
Linux Kernel: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/kernel/panic.c
Apple: https://opensource.apple.com/source/Libc/Libc-1439.40.11/sys/OpenBSD/stack_protector.c.auto.html

Link: https://gcc.gnu.org/onlinedocs/gccint/Stack-Smashing-Protection.html#Stack-Smashing-Protection

This will later help us diagnose functions that fall through to other
functions vs end in calls to functions that are noreturn.

Reviewed By: efriedma

Differential Revision: https://reviews.llvm.org/D146339