Uses alignas for portability in dynamic_annotations.h (#947)

Fixes an MSVC compiler error when address sanitizers are enabled
diff --git a/absl/base/dynamic_annotations.h b/absl/base/dynamic_annotations.h
index 880cbf6..03d7096 100644
--- a/absl/base/dynamic_annotations.h
+++ b/absl/base/dynamic_annotations.h
@@ -471,7 +471,7 @@
   __sanitizer_annotate_contiguous_container(beg, end, old_mid, new_mid)
 #define ABSL_ADDRESS_SANITIZER_REDZONE(name) \
   struct {                                   \
-    char x[8] __attribute__((aligned(8)));   \
+    alignas(8) char x[8];                    \
   } name
 
 #else