A cleanup for MCOL-4064 Make JOIN collation aware

A non-JOIN condition like `WHERE c1=c2` (with c1 and c2 being columns of the
same table) was not collation-aware yet after the main patches for MCOL-4064.

Additionally fixing StrFilterCmd::compare*() to address this.

datatypes/mcs_string.h

@@ -0,0 +1,45 @@
+/* Copyright (C) 2020 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 Foundation; version 2 of
+   the License.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program; if not, write to the Free Software
+   Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston,
+   MA 02110-1301, USA. */
+
+
+#ifndef MCS_DATATYPES_STRING_H
+#define MCS_DATATYPES_STRING_H
+
+#include "conststring.h"
+
+namespace datatypes
+{
+
+class TCharShort
+{
+    int64_t mValue;
+public:
+    TCharShort(int64_t value)
+        :mValue(value)
+    { }
+    explicit operator utils::ConstString() const
+    {
+      utils::ConstString res = utils::ConstString((const char *) &mValue, 8);
+      return res.rtrimZero();
+    }
+};
+
+
+} // namespace datatypes
+
+
+#endif // MCS_DATATYPES_STRING_H