c++: parse error with -std=c++14 -fconcepts [PR116071]

c++: parse error with -std=c++14 -fconcepts [PR116071]

cp_parser_simple_type_specifier tries a variety of different things that
might qualify as a user-defined type: an actual type-name, a constrained
auto, a CTAD placeholder.  In a context where a type-specifier is optional,
this is all tentative.  With -std=c++14 -fconcepts, we try type-name and
constrained auto in sub-tentative parses, and when we run out of things to
try we haven't found anything but also haven't failed the outer tentative
parse, so parse_definitely succeeds, discarding the nested-name-specifier.

Fixed by failing if we didn't find anything.

I said in r14-3203 that we should disable this combination of flags if
further problems arise, but this seems like a more general problem that only
happened to occur with just this combination of flags.  So it lives on.

	PR c++/116071

gcc/cp/ChangeLog:

	* parser.cc (cp_parser_simple_type_specifier): Call
	cp_parser_simulate_error if nothing worked.

gcc/testsuite/ChangeLog:

	* g++.dg/parse/pr116071.C: New test.