MDEV-5816: Stored programs: validation of stored program statements

This is the prerequisite patch to move the sp_instr class and classes derived
from it into the files sp_instr.cc/sp_instr.h. The classes sp_lex_cursor and
sp_lex_keeper are also moved to the files files sp_instr.cc/sp_instr.h.

Additionally,
  * all occurrences of macroses NULL, FALSE, TRUE are replaced
    with the corresponding C++ keywords nullptr, false, true.
  * the keyword 'override' is added in and the keyword 'virtual' is removed
    from signatures of every virtual method implemented in classes derived
    from the base class sp_instr.
  * the keyword 'final' is added into declaration of the class sp_lex_keeper
    since this class shouldn't have a derived class by design.
  * the function cmp_rqp_locations is made static since it is not called
    outside the file sp_instr.cc.
  * the function subst_spvars() is moved into the file sp_instr.cc since this
    function used only by the method sp_instr_stmt::execute

libmysqld/CMakeLists.txt

@@ -151,6 +151,7 @@ SET(SQL_EMBEDDED_SOURCES emb_qcache.cc libmysqld.c lib_sql.cc
            ../sql/xa.cc
            ../sql/json_table.cc
            ../sql/opt_histogram_json.cc
+           ../sql/sp_instr.cc
            ${GEN_SOURCES}
            ${MYSYS_LIBWRAP_SOURCE}
 )

sql/CMakeLists.txt

@@ -165,6 +165,7 @@ SET (SQL_SOURCE
                rpl_gtid.cc rpl_parallel.cc
                semisync.cc semisync_master.cc semisync_slave.cc
                semisync_master_ack_receiver.cc
+               sp_instr.cc
                sql_schema.cc
                lex_charset.cc charset_collations.cc
                sql_type.cc sql_mode.cc sql_type_json.cc