Make GrVkRenderPass::AttachmentFlags enum unsigned

These are flags and so are logically unsigned. This fixes an issue where
ASAN detects that the bitwise negation of of all listed enum values will
not fit in underlying enum type.

Change-Id: Idb993f29bdac36ef8960392b89a748f708dae520
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/517878
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Ben Wagner <bungeman@google.com>

src/gpu/vk/GrVkRenderPass.h

@@ -80,7 +80,7 @@ public:
         uint32_t       fAttachmentCount;
     };
 
-    enum AttachmentFlags {
+    enum AttachmentFlags : uint32_t {
         kColor_AttachmentFlag = 0x1,
         kStencil_AttachmentFlag = 0x2,
         kResolve_AttachmentFlag = 0x4,