PR c++/88795 reports an ICE building a function_type for a deduction guide
when the substitution into the function signature fails, due to an
error_mark_node being returned from tsubst_arg_types but not being checked
for.  This error_mark_node gets used as the TYPE_ARG_TYPES, leading to
ICEs in various places that assume this is a TREE_LIST.

This patch checks the result of tsubst_arg_types and propagates the failure
if it returns error_mark_node.  It also adds an assertion to
build_function_type, to fail faster if passed in error_mark_node.

gcc/cp/ChangeLog:
	PR c++/88795
	* pt.c (build_deduction_guide): Bail out if tsubst_arg_types
	fails.

gcc/testsuite/ChangeLog:
	PR c++/88795
	* g++.dg/template/pr88795.C: New test.

gcc/ChangeLog:
	PR c++/88795
	* tree.c (build_function_type): Assert that arg_types is not
	error_mark_node.

From-SVN: r267957