Split class Database_qualified_name

Class Database_qualified_name is currently used for two purposes when handling package routines such as `db.pkg.func`:

• to store the `db.pkg` pair
• to store the `pkg.func` pair

The data type of Database_qualified_name::m_name will be changed from LEX_CSTRING to Lex_ident_db soon, so it will contain a validated and normalized database name, which means lower-cased if lower-case-table-names says so.

Therefore, Database_qualified_name won't be able to store `pkg.func` pairs any more - the `pkg` part must not depend on lower-case-table-names.

Let's add a new class Identifier_chain2 and move the following methods from Database_qualified_name to Identifier_chain2, as follows:

class Identifier_chain2

{

  LEX_CSTRING m_name[2];

public:

  Identifier_chain2(const LEX_CSTRING &a, const LEX_CSTRING &b)

   :m_name{a, b}

  { }

  // These methods were in Database_qualified_name originally:

  static Identifier_chain2 split(const LEX_CSTRING &txt);

  size_t make_qname(char *dst, size_t dstlen) const;

  bool make_qname(MEM_ROOT *mem_root, LEX_CSTRING *dst) const;

};

Also, let's change the data type of the following variables from Database_qualified_name to Identifier_chain2:

• q_pkg_proc in LEX::call_statement_start()
• q_pkg_func in LEX::make_item_func_call_generic()