MDEV-31978 Turn ok_for_lower_case_names() to a method in Lex_ident_fs

- Changing the global function ok_for_lower_case_names()
  into a method in class Lex_ident_fs.

- Changing a few functions/methods to get the database name
  as a "const LEX_CSTRING" instead of a "const char *".
  All these functions/methods use ok_for_lower_case_names()
  inside. This change helps to avoid new strlen() calls, and also
  removes a few old strlen() calls.

sql/sql_class.h

@@ -7844,7 +7844,7 @@ public:
             (int) m_db.length, (m_db.length ? m_db.str : ""),
             dot, ".",
             (int) m_name.length, m_name.str);
-    DBUG_SLOW_ASSERT(ok_for_lower_case_names(m_db.str));
+    DBUG_SLOW_ASSERT(Lex_ident_fs(m_db).ok_for_lower_case_names());
     return false;
   }