fixes # 25846; Uses decltype(auto) type when declaring temporal

[demotomohiro]

proc testVarRet(x: var int): var int = x

proc testProcTypeParam(prc: proc (x: var int): var int {.nimcall.}) =
  var x = 123
  prc(x) = 321
  echo x

testProcTypeParam(testVarRet)

Nim cpp generates C++ code like following from above Nim code:

#include <iostream>

typedef int& (tyProc)(int& x_p0);

int* testVarRet(int& x) {
  return &x;
}

void testProcTypeParam(tyProc prc) {
  int x = 123;
  auto T1 = prc(x);
  T1 = 321;
  std::cout << x << std::endl;
}

int main()
{
  testProcTypeParam(testVarRet);
}

But the type T1 is int, not int& so the value of local variable x wasn't changed.
This PR changes type of temporal variables from auto to decltype(auto) so that type of temporal variables become reference type when they are initialized with a function with reference return type.

Following C++ shows how auto and decltype(auto) in variable declarations works differently:

#include <iostream>

int& foo(int& x) {
  return x;
}

int main()
{
    int x = 100;
    auto y = foo(x);            // y is int
    decltype(auto) z = foo(x);  // z is int&
    y = 123;
    std::cout << x << ", " << y << std::endl;   // 100, 123
    z = 456;
    std::cout << x << ", " << z << std::endl;   // 456, 456

    int a = 1;
    decltype(auto) b = a;       // b is int
    decltype(auto) c = (a);     // c is int&
    b = 2;
    std::cout << a << ", " << b << std::endl;   // 1, 2
    c = 3;
    std::cout << a << ", " << c << std::endl;   // 3, 3
}

ref #25846

[demotomohiro]

Need to fix #25870 to pass CI tests.