Apply clang-tidy to remove empty constructors / destructors

This patch is the result of running
run-clang-tidy -fix -header-filter=.* -checks='-*,modernize-use-equals-default' .

Code style changes have been done on top. The result of this change
leads to the following improvements:

1. Binary size reduction.
* For a -DBUILD_CONFIG=mysql_release build, the binary size is reduced by
  ~400kb.
* A raw -DCMAKE_BUILD_TYPE=Release reduces the binary size by ~1.4kb.

2. Compiler can better understand the intent of the code, thus it leads
   to more optimization possibilities. Additionally it enabled detecting
   unused variables that had an empty default constructor but not marked
   so explicitly.

   Particular change required following this patch in sql/opt_range.cc

   result_keys, an unused template class Bitmap now correctly issues
   unused variable warnings.

   Setting Bitmap template class constructor to default allows the compiler
   to identify that there are no side-effects when instantiating the class.
   Previously the compiler could not issue the warning as it assumed Bitmap
   class (being a template) would not be performing a NO-OP for its default
   constructor. This prevented the "unused variable warning".

1 files changed, 3 insertions, 3 deletions

diff --git a/sql/set_var.h b/sql/set_var.h
index 56de00221d8..705751b8296 100644
--- a/sql/set_var.h
+++ b/sql/set_var.h
@@ -99,7 +99,7 @@ public:
           on_check_function on_check_func, on_update_function on_update_func,
           const char *substitute);
 
-  virtual ~sys_var() {}
+  virtual ~sys_var() = default;
 
   /**
     All the cleanup procedures should be performed here
@@ -269,8 +269,8 @@ protected:
 class set_var_base :public Sql_alloc
 {
 public:
-  set_var_base() {}
-  virtual ~set_var_base() {}
+  set_var_base() = default;
+  virtual ~set_var_base() = default;
   virtual int check(THD *thd)=0;           /* To check privileges etc. */
   virtual int update(THD *thd)=0;                  /* To set the value */
   virtual int light_check(THD *thd) { return check(thd); }   /* for PS */