Reduce the size of Lex_length_and_dec_st from 16 to 8

We want to use Bison stack more efficiently and pass more structures through the stack instead of using Lex members.

In particular, we want to get rid of Lex->charset. This change is really overdue.

To be able to do this, we want to pass through the Bison stack a structure with data type handler and its basic attributes, consisting of the following four members:

struct {

  const Type_handler *handler;

  CHARSET_INFO *charset;

  Lex_length_and_dec_st length_and_dec;

};

This structure should fit into the Bison variable limitation (see sql_yacc.yy):

/* avoid unintentional %union size increases, it's what a parser stack made of */

static_assert(sizeof(YYSTYPE) == sizeof(void*)*2+8, "%union size check");

Currently this structure would not fit because Lex_length_and_dec_st consists of two pointers and therefore occupies 16 bytes (on 64bit platforms):

struct Lex_length_and_dec_st

{

private:

  const char *m_length;

  const char *m_dec;

public:

Let's reduce the size of Lex_length_and_dec_st from current 16 bytes as follows:

struct Lex_length_and_dec_st

{

private:

  uint32 m_length;

  uint8  m_dec;

  bool   m_has_explicit_length:1;

  bool   m_has_explicit_dec:1;

public:

After this change, the future structure consisting of the four fields (mentioned above) will occupy 24 bytes:

struct {

  const Type_handler *handler;          // 8 bytes

  CHARSET_INFO *charset;                // 8 bytes

  Lex_length_and_dec_st length_and_dec; // 8 bytes

};

Note, this task is only about changing Lex_length_and_dec_st. The new structure (with handler, charset, length and dec) will be added later, under terms of a separate task.