Remove the most annoying clang-cl warnings

Fix "consistent override", and "unused private field" warnings
in popular header files.

sql/field.h

@@ -84,7 +84,6 @@ class Conv_param
 
 class Conv_source: public Type_handler_hybrid_field_type
 {
-  enum_field_types m_type;
   uint16 m_metadata;
   CHARSET_INFO *m_cs;
 public:

sql/item_cmpfunc.h

@@ -2792,7 +2792,6 @@ class Regexp_processor_pcre
   bool m_conversion_is_needed;
   bool m_is_const;
   int m_library_flags;
-  CHARSET_INFO *m_data_charset;
   CHARSET_INFO *m_library_charset;
   String m_prev_pattern;
   int m_pcre_exec_rc;
@@ -2809,7 +2808,6 @@ class Regexp_processor_pcre
   Regexp_processor_pcre() :
     m_pcre(NULL), m_conversion_is_needed(true), m_is_const(0),
     m_library_flags(0),
-    m_data_charset(&my_charset_utf8mb3_general_ci),
     m_library_charset(&my_charset_utf8mb3_general_ci)
   {
     m_pcre_extra.flags= PCRE_EXTRA_MATCH_LIMIT_RECURSION;

sql/item_jsonfunc.h

@@ -127,9 +127,9 @@ class Item_func_json_value: public Item_str_func,
 public:
   Item_func_json_value(THD *thd, Item *js, Item *i_path):
     Item_str_func(thd, js, i_path) {}
-  const char *func_name() const { return "json_value"; }
-  bool fix_length_and_dec();
-  String *val_str(String *to)
+  const char *func_name() const override { return "json_value"; }
+  bool fix_length_and_dec() override ;
+  String *val_str(String *to) override
   {
     null_value= Json_path_extractor::extract(to, args[0], args[1],
                                              collation.collation);
@@ -140,7 +140,7 @@ class Item_func_json_value: public Item_str_func,
   {
     return je->check_and_get_value_scalar(res, error);
   }
-  Item *get_copy(THD *thd)
+  Item *get_copy(THD *thd) override
   { return get_item_copy<Item_func_json_value>(thd, this); }
 };
 
@@ -151,9 +151,9 @@ class Item_func_json_query: public Item_json_func,
 public:
   Item_func_json_query(THD *thd, Item *js, Item *i_path):
     Item_json_func(thd, js, i_path) {}
-  const char *func_name() const { return "json_query"; }
-  bool fix_length_and_dec();
-  String *val_str(String *to)
+  const char *func_name() const override { return "json_query"; } 
+  bool fix_length_and_dec() override;
+  String *val_str(String *to) override
   {
     null_value= Json_path_extractor::extract(to, args[0], args[1],
                                              collation.collation);
@@ -164,7 +164,7 @@ class Item_func_json_query: public Item_json_func,
   {
     return je->check_and_get_value_complex(res, error);
   }
-  Item *get_copy(THD *thd)
+  Item *get_copy(THD *thd) override
   { return get_item_copy<Item_func_json_query>(thd, this); }
 };