c++: ill-formed constexpr function [PR113360]

c++: ill-formed constexpr function [PR113360]

If we already gave an error while parsing a function, we don't also need to
try to explain what's wrong with it when we later try to use it in a
constant-expression.  In the new testcase explain_invalid_constexpr_fn
couldn't find anything still in the function to complain about, so it said
because: followed by nothing.

We still try to constant-evaluate it to reduce error cascades, but we
shouldn't complain if it doesn't work very well.

This flag is similar to CLASSTYPE_ERRONEOUS that I added a while back.

	PR c++/113360

gcc/cp/ChangeLog:

	* cp-tree.h (struct language_function): Add erroneous bit.
	* constexpr.cc (explain_invalid_constexpr_fn): Return if set.
	(cxx_eval_call_expression): Quiet if set.
	* parser.cc (cp_parser_function_definition_after_declarator)
	* pt.cc (instantiate_body): Set it.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp23/constexpr-nonlit18.C: Remove redundant message.
	* g++.dg/cpp1y/constexpr-diag2.C: New test.
	* g++.dg/cpp1y/pr63996.C: Adjust expected errors.
	* g++.dg/template/explicit-args6.C: Likewise.
	* g++.dg/cpp0x/constexpr-ice21.C: Likewise.