Bring back move implementation of async_msg move ctor and assignment since VS doesn't support it

include/spdlog/details/async_log_helper.h

@@ -64,9 +64,21 @@ class async_log_helper
         async_msg() = default;
         ~async_msg() = default;
 
-        async_msg(async_msg&& other) = default;
-        async_msg& operator=(async_msg&& other) = default;
+        async_msg(async_msg&& other) SPDLOG_NOEXCEPT:
+            logger_name(std::move(other.logger_name)),
+            level(std::move(other.level)),
+            time(std::move(other.time)),
+            txt(std::move(other.txt))
+        {}
 
+        async_msg& operator=(async_msg&& other) SPDLOG_NOEXCEPT
+        {
+            logger_name = std::move(other.logger_name);
+            level = other.level;
+            time = std::move(other.time);
+            txt = std::move(other.txt);
+            return *this;
+        }
         // never copy or assign. should only be moved..
         async_msg(const async_msg&) = delete;
         async_msg& operator=(async_msg& other) = delete;