c++: -Wtemplate-body and tentative parsing [PR120575]

c++: -Wtemplate-body and tentative parsing [PR120575]

Here we were asserting non-zero errorcount, which is not the case if the
parse error was reduced to a warning (or silenced) in a template body.  So
check seen_error instead.

	PR c++/120575
	PR c++/116064

gcc/cp/ChangeLog:

	* parser.cc (cp_parser_abort_tentative_parse): Check seen_error
	instead of errorcount.

gcc/testsuite/ChangeLog:

	* g++.dg/template/permissive-error3.C: New test.