c++: fix ICE with xobj in destructor [PR113340]

Here we crash in maybe_retrofit_in_chrg on an invalid dtor
with explicit this.  Such member functions do not get converted
to METHOD_TYPE.  If a dtor gets parameters, we reset arg_types
to void_list_node in grokdeclarator.  This results in m_r_in_c
receiving:
  void <T8d> (void)
and crashing on
  parms = DECL_CHAIN (DECL_ARGUMENTS (fn));

This patch avoids the ICE by resetting is_xobj_member_function after
emitting the error.  Then m_r_in_c gets
  void S::<T40b> (struct S *)
which does not cause a crash.

	PR c++/113340

gcc/cp/ChangeLog:

	* decl.cc (grokdeclarator) <case cdk_function>: Clear
	is_xobj_member_function in case of an error.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp23/explicit-obj-diagnostics9.C: New test.