MDEV-18677 clang-cl 7 fails to compile innodb

ib_counter_element_t: Declare as struct, not union.

Based on patch by Vladislav Vaintroub

storage/innobase/include/ut0counter.h

 /*****************************************************************************
 
 Copyright (c) 2012, 2015, Oracle and/or its affiliates. All Rights Reserved.
-Copyright (c) 2017, 2018, MariaDB Corporation.
+Copyright (c) 2017, 2019, MariaDB Corporation.
 
 This program is free software; you can redistribute it and/or modify it under
 the terms of the GNU General Public License as published by the Free Software
@@ -105,10 +105,16 @@ struct ib_counter_t {
 	}
 
 private:
-	/** Atomic which occupies whole CPU cache line */
-	union ib_counter_element_t {
-		std::atomic<Type> value;
-		byte padding[CACHE_LINE_SIZE];
+	/** Atomic which occupies whole CPU cache line.
+	Note: We rely on the default constructor of std::atomic and
+	do not explicitly initialize the contents. This works for us,
+	because ib_counter_t is only intended for usage with global
+	memory that is allocated from the .bss and thus guaranteed to
+	be zero-initialized by the run-time environment.
+	@see srv_stats
+	@see rw_lock_stats */
+	struct ib_counter_element_t {
+		MY_ALIGNED(CACHE_LINE_SIZE) std::atomic<Type> value;
 	};
 	static_assert(sizeof(ib_counter_element_t) == CACHE_LINE_SIZE, "");