Use C++17's [[nodiscard]] instead of WARN_UNUSED_RESULT

No functionality change. Also remove some header guards along the way.

Test: m
Change-Id: I1afdcaea95a3dd56f392c4e61d7670f43615792a
diff --git a/server/NetlinkCommands.cpp b/server/NetlinkCommands.cpp
index 7af33fe..acefa8e 100644
--- a/server/NetlinkCommands.cpp
+++ b/server/NetlinkCommands.cpp
@@ -65,17 +65,25 @@
     return response.err.error;  // Netlink errors are negative errno.
 }
 
+// Disable optimizations in ASan build.
+// ASan reports an out-of-bounds 32-bit(!) access in the first loop of the function (over iov[]).
+// TODO: verify if this bug is still present.
+#ifdef __clang__
+#if __has_feature(address_sanitizer)
+#define OPTNONE [[clang::optnone]]
+#endif
+#endif
+
+#ifndef OPTNONE
+#define OPTNONE /* nop */
+#endif
+
 // Sends a netlink request and possibly expects an ack.
 // |iov| is an array of struct iovec that contains the netlink message payload.
 // The netlink header is generated by this function based on |action| and |flags|.
 // Returns -errno if there was an error or if the kernel reported an error.
-#ifdef __clang__
-#if __has_feature(address_sanitizer)
-__attribute__((optnone))
-#endif
-#endif
-WARN_UNUSED_RESULT int sendNetlinkRequest(uint16_t action, uint16_t flags, iovec* iov, int iovlen,
-                                          const NetlinkDumpCallback *callback) {
+OPTNONE int sendNetlinkRequest(uint16_t action, uint16_t flags, iovec* iov, int iovlen,
+                               const NetlinkDumpCallback* callback) {
     nlmsghdr nlmsg = {
         .nlmsg_type = action,
         .nlmsg_flags = flags,
@@ -146,8 +154,8 @@
     return 0;
 }
 
-WARN_UNUSED_RESULT int rtNetlinkFlush(uint16_t getAction, uint16_t deleteAction,
-                                     const char *what, const NetlinkDumpFilter& shouldDelete) {
+int rtNetlinkFlush(uint16_t getAction, uint16_t deleteAction, const char* what,
+                   const NetlinkDumpFilter& shouldDelete) {
     // RTM_GETxxx is always RTM_DELxxx + 1, see <linux/rtnetlink.h>.
     if (getAction != deleteAction + 1) {
         ALOGE("Unknown flush type getAction=%d deleteAction=%d", getAction, deleteAction);
@@ -201,7 +209,7 @@
     return ret;
 }
 
-uint32_t getRtmU32Attribute(const nlmsghdr *nlh, int attribute) {
+uint32_t getRtmU32Attribute(const nlmsghdr* nlh, int attribute) {
     uint32_t rta_len = RTM_PAYLOAD(nlh);
     rtmsg *msg = reinterpret_cast<rtmsg *>(NLMSG_DATA(nlh));
     rtattr *rta = reinterpret_cast<rtattr *> RTM_RTA(msg);