SELECT_LEX objects that are "fake_select_lex" (i.e read UNION output)
used both INT_MAX and UINT_MAX as select_number.
- mysql_explain_union() assigned UINT_MAX
- st_select_lex_unit::add_fake_select_lex assigned INT_MAX

This didn't matter initially (before EXPLAIN FORMAT=JSON), because the
code  had no checks for this value.

EXPLAIN FORMAT=JSON and later other features did introduce checks for
select_number values. The check had to check for two constants and
looked really confusing.

This patch joins the two constants into one - FAKE_SELECT_LEX_ID.