Clang 14 introduces support for compiletime_assert(). Update the
compile-time warning regex to catch Clang's variant of the warning text
in preparation for Clang supporting CONFIG_FORTIFY_SOURCE.

Cc: Nick Desaulniers <ndesaulniers@google.com>
Cc: linux-hardening@vger.kernel.org
Cc: llvm@lists.linux.dev
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Link: https://lore.kernel.org/lkml/YfbtQKtpyAM1hHiC@dev-arch.archlinux-ax161
Signed-off-by: Kees Cook <keescook@chromium.org>

While the run-time testing of FORTIFY_SOURCE is already present in
LKDTM, there is no testing of the expected compile-time detections. In
preparation for correctly supporting FORTIFY_SOURCE under Clang, adding
additional FORTIFY_SOURCE defenses, and making sure FORTIFY_SOURCE
doesn't silently regress with GCC, introduce a build-time test suite that
checks each expected compile-time failure condition.

As this is relatively backwards from standard build rules in the
sense that a successful test is actually a compile _failure_, create
a wrapper script to check for the correct errors, and wire it up as
a dummy dependency to lib/string.o, collecting the results into a log
file artifact.

Signed-off-by: Kees Cook <keescook@chromium.org>