c++: Don't reject pointer to virtual method during constant evaluation [PR117615]

c++: Don't reject pointer to virtual method during constant evaluation [PR117615]

We currently reject the following valid code:

=== cut here ===
struct Base {
    virtual void doit (int v) const {}
};
struct Derived : Base {
    void doit (int v) const {}
};
using fn_t = void (Base::*)(int) const;
struct Helper {
    fn_t mFn;
    constexpr Helper (auto && fn) : mFn(static_cast<fn_t>(fn)) {}
};
void foo () {
    constexpr Helper h (&Derived::doit);
}
=== cut here ===

The problem is that since r6-4014-gdcdbc004d531b4, &Derived::doit is
represented with an expression with type pointer to method and using an
INTEGER_CST (here 1), and that cxx_eval_constant_expression rejects any
such expression with a non-null INTEGER_CST.

This patch uses the same strategy as r12-4491-gf45610a45236e9 (fix for
PR c++/102786), and simply lets such expressions go through.

	PR c++/117615

gcc/cp/ChangeLog:

	* constexpr.cc (cxx_eval_constant_expression): Don't reject
	INTEGER_CSTs with type POINTER_TYPE to METHOD_TYPE.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/constexpr-virtual22.C: New test.