c++: Hard error with tentative parse of declaration [PR88754]

c++: Hard error with tentative parse of declaration [PR88754]

In the testcase for this PR, we try to parse the statement

  A(value<0>());

first tentatively as a declaration (with a parenthesized declarator), and during
this tentative parse we end up issuing a hard error from
cp_parser_check_template_parameters about its invalidness as a declaration.

Rather than issuing a hard error, it seems we should instead simulate an error
since we're parsing tentatively.  This would then allow cp_parser_statement to
recover and successfully parse the statement as an expression-statement instead.

gcc/cp/ChangeLog:

	PR c++/88754
	* parser.c (cp_parser_check_template_parameters): Before issuing a hard
	error, first try simulating an error instead.

gcc/testsuite/ChangeLog:

	PR c++/88754
	* g++.dg/parse/ambig10.C: New test.