Move functions case_stmt_xxx and add_select_to_union_list as methods to LEX

The task MDEV-10142 will introduce the second *.yy file, with Oracle PL/SQL grammar.
Before we add the second *.yy file, we should move the functions defined in sql_yacc.yy as methods to lex, to avoid code duplication.
Under terms of this tasks we'll move:

int case_stmt_action_expr(LEX *, Item* expr);

int case_stmt_action_when(LEX *, Item *when, bool simple);

int case_stmt_action_then(LEX *);

bool add_select_to_union_list(LEX *,bool is_union_distinct,  bool is_top_level);