c++: Fix endless errors on invalid requirement seq [PR97742]

c++: Fix endless errors on invalid requirement seq [PR97742]

As the testcase shows, if we reach CPP_EOF during parsing of requirement
sequence, we end up with endless loop where we always report invalid
requirement expression, don't consume any token (as we are at eof) and
repeat.

This patch stops the loop when we reach CPP_EOF.

2021-02-12  Jakub Jelinek  <jakub@redhat.com>

	PR c++/97742
	* parser.c (cp_parser_requirement_seq): Stop iterating after reaching
	CPP_EOF.

	* g++.dg/cpp2a/concepts-requires24.C: New test.