kasan: add compiler support for clang

Patch series "kasan: support alloca, LLVM", v4.

This patch (of 5):

For now we can hard-code ASAN ABI level 5, since historical clang builds
can't build the kernel anyway.  We also need to emulate gcc's
__SANITIZE_ADDRESS__ flag, or memset() calls won't be instrumented.

Link: http://lkml.kernel.org/r/20171204191735.132544-2-paullawrence@google.com
Signed-off-by: Greg Hackmann <ghackmann@google.com>
Signed-off-by: Paul Lawrence <paullawrence@google.com>
Acked-by: Andrey Ryabinin <aryabinin@virtuozzo.com>
Cc: Alexander Potapenko <glider@google.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Masahiro Yamada <yamada.masahiro@socionext.com>
Cc: Matthias Kaehlcke <mka@chromium.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/compiler-clang.h

@@ -19,3 +19,11 @@
 
 #define randomized_struct_fields_start	struct {
 #define randomized_struct_fields_end	};
+
+/* all clang versions usable with the kernel support KASAN ABI version 5 */
+#define KASAN_ABI_VERSION 5
+
+/* emulate gcc's __SANITIZE_ADDRESS__ flag */
+#if __has_feature(address_sanitizer)
+#define __SANITIZE_ADDRESS__
+#endif