C++: Fix ICE when adding overloaded operator via using_decl (PR c++/88699)

C++: Fix ICE when adding overloaded operator via using_decl (PR c++/88699)

PR c++/88699 reports an ICE within this assertion in add_method:

  gcc_assert (!current_fns || !DECL_DESTRUCTOR_P (method));

when adding an overloaded operator to a class via a using_decl, due to
DECL_DESTRUCTOR_P requiring a FUNCTION_DECL, but "method" being a
USING_DECL.

This patch weakens the assertion to avoid testing DECL_DESTRUCTOR_P
for the case where "via_using" is true, fixing the ICE.

gcc/cp/ChangeLog:
	PR c++/88699
	* class.c (add_method): Don't use DECL_DESTRUCTOR_P on
	USING_DECLs.

gcc/testsuite/ChangeLog:
	PR c++/88699
	* g++.dg/template/pr88699.C: New test.

From-SVN: r268041