C++ concepts: fix ICE with requires on dtors (PR c++/89036)

C++ concepts: fix ICE with requires on dtors (PR c++/89036)

PR c++/89036 reports an ICE due to this assertion failing

1136	  /* A class should never have more than one destructor.  */
1137	  gcc_assert (!current_fns || via_using || !DECL_DESTRUCTOR_P (method));

on this template with a pair of dtors, with
mutually exclusive "requires" clauses:

template<typename T>
struct Y {
    ~Y() requires(true) = default;
    ~Y() requires(false) {}
};

Nathan introduced this assertion as part of:

  ca9219bf18c68a001d62ecb981bc9176b0feaf12 (aka r251340):
    2017-08-24  Nathan Sidwell  <nathan@acm.org>
       Conversion operators kept on single overload set

which, amongst other changes to add_method had this:
     /* A class should never have more than one destructor.  */
  -  if (current_fns && DECL_MAYBE_IN_CHARGE_DESTRUCTOR_P (method))
  -    return false;
  +  gcc_assert (!current_fns || !DECL_DESTRUCTOR_P (method));

The following patch drops the assertion (I already had to generalize
the assertion in r268041 to fix PR c++/88699).

gcc/cp/ChangeLog:
	PR c++/89036
	* class.c (add_method): Drop destructor assertion.

gcc/testsuite/ChangeLog:
	PR c++/89036
	* g++.dg/concepts/pr89036.C: New test.

From-SVN: r268847