c++: Don't ICE due to artificial constructor parameters [PR116722]

The following code triggers an ICE

=== cut here ===
class base {};
class derived : virtual public base {
public:
  template<typename Arg> constexpr derived(Arg) {}
};
int main() {
  derived obj(1.);
}
=== cut here ===

The problem is that cxx_bind_parameters_in_call ends up attempting to
convert a REAL_CST (the first non artificial parameter) to INTEGER_TYPE
(the type of the __in_chrg parameter), which ICEs.

This patch changes cxx_bind_parameters_in_call to return early if it's
called with a *structor that has an __in_chrg or __vtt_parm parameter
since the expression won't be a constant expression.

Note that in the test case, the constructor is not constexpr-suitable,
however it's OK since it's a template according to my read of paragraph
(3) of [dcl.constexpr].

	PR c++/116722

gcc/cp/ChangeLog:

	* constexpr.cc (cxx_bind_parameters_in_call): Leave early for
	{con,de}structors of classes with virtual bases.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/constexpr-ctor22.C: New test.