c++: Improve diagnostics for template args terminated with >= or >>= [PR104319]

c++: Improve diagnostics for template args terminated with >= or >>= [PR104319]

As mentioned in the PR, for C++98 we have diagnostics that expect
>> terminating template arguments to be a mistake for > > (C++11
said it has to be treated that way), while if user trying to spare the
spacebar doesn't separate > from following = or >> from following =,
the diagnostics is confusing, while clang suggests adding space in between.

The following patch does that for >= and >>= too.

For some strange reason the error recovery emits further errors,
not really sure what's going on because I overwrite the token->type
like the code does for the C++11 >> case or for the C++98 >> cases,
but at least the first error is nicer (well, for the C++98 nested
template case and >>= I need to overwrite it to > and so the = is lost,
so perhaps some follow-up errors are needed for that case).

2022-04-29  Jakub Jelinek  <jakub@redhat.com>

	PR c++/104319
	* parser.cc (cp_parser_template_argument): Treat >= like C++98 >>
	after a type id by setting maybe_type_id and aborting tentative
	parse.
	(cp_parser_enclosed_template_argument_list): Handle
	CPP_GREATER_EQ like misspelled CPP_GREATER CPP_RQ and
	CPP_RSHIFT_EQ like misspelled CPP_GREATER CPP_GREATER_EQ
	or CPP_RSHIFT CPP_EQ or CPP_GREATER CPP_GREATER CPP_EQ.
	(cp_parser_next_token_ends_template_argument_p): Return true
	also for CPP_GREATER_EQ and CPP_RSHIFT_EQ.

	* g++.dg/parse/template28.C: Adjust expected diagnostics.
	* g++.dg/parse/template30.C: New test.