c++: ICE when xobj is not the first parm [PR113389]

c++: ICE when xobj is not the first parm [PR113389]

In grokdeclarator/cdk_function the comment says that the find_xobj_parm
lambda clears TREE_PURPOSE so that we can correctly detect an xobj that
is not the first parameter.  That's all good, but we should also clear
the TREE_PURPOSE once we've given the error, otherwise we crash later in
check_default_argument because the 'this' TREE_PURPOSE lacks a type.

	PR c++/113389

gcc/cp/ChangeLog:

	* decl.cc (grokdeclarator) <case cdk_function>: Set TREE_PURPOSE to
	NULL_TREE when emitting an error.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp23/explicit-obj-diagnostics10.C: New test.