Deprecated copy warning with GCC-12

[voskh0d]

When compiling with g++ (GCC) 12.2.0, ac_bitref functions and ac_int::operator[] will produce a deprecated copy warning. Here an example on how to reproduce them:

#include "ac_int.h"

int main() {
  ac_int<3,false> a;
  a[0] = 1;
  return 0;
}

Compiled with g++ -Wall -Wextra -std=c++17 -I . t.cc

Produces:

ac_int.h: In instantiation of 'ac_int<W, S>::ac_bitref ac_int<W, S>::operator[](int) [with int W = 3; bool S = false]':
t.cc:5:6:   required from here
ac_int.h:2603:12: warning: implicitly-declared 'constexpr ac_int<3, false>::ac_bitref::ac_bitref(const ac_int<3, false>::ac_bitref&)' is deprecated [-Wdeprecated-copy]
 2603 |     return bvh;
      |            ^~~
ac_int.h:2588:22: note: because 'ac_int<3, false>::ac_bitref' has user-provided 'ac_int<W, S>::ac_bitref ac_int<W, S>::ac_bitref::operator=(const ac_int<W, S>::ac_bitref&) [with int W = 3; bool S = false]'
 2588 |     inline ac_bitref operator = ( const ac_bitref &val ) {
      |                      ^~~~~~~~
ac_int.h: In instantiation of 'ac_int<W, S>::ac_bitref ac_int<W, S>::ac_bitref::operator=(int) [with int W = 3; bool S = false]':
t.cc:5:10:   required from here
ac_int.h:2582:15: warning: implicitly-declared 'constexpr ac_int<3, false>::ac_bitref::ac_bitref(const ac_int<3, false>::ac_bitref&)' is deprecated [-Wdeprecated-copy]
 2582 |       return *this;
      |               ^~~~
ac_int.h:2588:22: note: because 'ac_int<3, false>::ac_bitref' has user-provided 'ac_int<W, S>::ac_bitref ac_int<W, S>::ac_bitref::operator=(const ac_int<W, S>::ac_bitref&) [with int W = 3; bool S = false]'
 2588 |     inline ac_bitref operator = ( const ac_bitref &val ) {
      |                      ^~~~~~~~