Reland "Introduce RTC_NO_UNIQUE_ADDRESS."

This is a reland of f5e261aaf65cdf2eb903cdf40d651846be44f447

This CL disables RTC_NO_UNIQUE_ADDRESS on MSan builds since
there have been some issues.

Original change's description:
> Introduce RTC_NO_UNIQUE_ADDRESS.
>
> This macro introduces the possibility to suggest the compiler that a
> data member doesn't need an address different from other non static
> data members.
>
> The usage of a macro is to maintain portability since at the moment
> the attribute [[no_unique_address]] is only supported by clang
> with at least -std=c++11 but it should be supported by all the
> compilers starting from C++20.
>
> Bug: webrtc:11495
> Change-Id: I9f12b67b4422a2749649eaa6b004a67d5fd572d8
> Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/173331
> Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
> Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
> Cr-Commit-Position: refs/heads/master@{#32246}

Bug: webrtc:11495, webrtc:12218
Change-Id: I4e6c7cc37d3daffad2407c9a2acfa897fa5b426a
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/189968
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#32668}
diff --git a/rtc_tools/network_tester/packet_sender.h b/rtc_tools/network_tester/packet_sender.h
index ffc1118..c0ea2c1 100644
--- a/rtc_tools/network_tester/packet_sender.h
+++ b/rtc_tools/network_tester/packet_sender.h
@@ -18,6 +18,7 @@
 #include "rtc_base/constructor_magic.h"
 #include "rtc_base/ignore_wundef.h"
 #include "rtc_base/synchronization/sequence_checker.h"
+#include "rtc_base/system/no_unique_address.h"
 #include "rtc_base/task_queue.h"
 
 #ifdef WEBRTC_NETWORK_TESTER_PROTO
@@ -49,7 +50,7 @@
   void UpdateTestSetting(size_t packet_size, int64_t send_interval_ms);
 
  private:
-  SequenceChecker worker_queue_checker_;
+  RTC_NO_UNIQUE_ADDRESS SequenceChecker worker_queue_checker_;
   size_t packet_size_ RTC_GUARDED_BY(worker_queue_checker_);
   int64_t send_interval_ms_ RTC_GUARDED_BY(worker_queue_checker_);
   int64_t sequence_number_ RTC_GUARDED_BY(worker_queue_checker_);