c++: make build_throw SFINAE-friendly [PR98388]

Here the problem is that we give hard errors while substituting
template parameters during overload resolution of is_throwable
which has an invalid throw in decltype.

The backtrace shows that fn_type_unification -> instantiate_template
-> tsubst* passes complain=0 as expected, but build_throw doesn't
have a complain parameter.  So let's add one.  Also remove a redundant
local variable which I should have removed in my P2266 patch.

There's still one problem for which I opened <https://gcc.gnu.org/PR113853>.
We need to patch up treat_lvalue_as_rvalue_p and remove the dg-bogus.

Thanks to Patrick for notifying me of this PR.  This doesn't fully fix
113789; there I think I'll have to figure our why a candidate wasn't
discarded from the overload set.

	PR c++/98388

gcc/cp/ChangeLog:

	* coroutines.cc (coro_rewrite_function_body): Pass tf_warning_or_error
	to build_throw.
	(morph_fn_to_coro): Likewise.
	* cp-tree.h (build_throw): Adjust.
	* except.cc (expand_end_catch_block): Pass tf_warning_or_error to
	build_throw.
	(build_throw): Add a tsubst_flags_t parameter.  Use it.  Remove
	redundant variable.  Guard an inform call.
	* parser.cc (cp_parser_throw_expression): Pass tf_warning_or_error
	to build_throw.
	* pt.cc (tsubst_expr) <case THROW_EXPR>: Pass complain to build_throw.

libcc1/ChangeLog:

	* libcp1plugin.cc (plugin_build_unary_expr): Pass tf_error to
	build_throw.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/sfinae69.C: New test.