C++ PATCH for DR 1518 (c++/54835, c++/60417)

[Ville Voutilainen]

It seems to me that there's a discrepancy in handling explicit
default constructors. Based on my tests, this works:

struct X {explicit X() {}};

void f(X) {}

int main()
{
    f({});
}

However, if the explicit constructor is defaulted, gcc accepts the code:

struct X {explicit X() = default;};

void f(X) {}

int main()
{
    f({});
}