c++: Don't try to parse a function declaration as deduction guide [PR97663]

c++: Don't try to parse a function declaration as deduction guide [PR97663]

While these function declarations have NULL decl_specifiers->type,
they have still type specifiers specified from which the default int
in the return type is added, so we shouldn't try to parse those as
deduction guides.

2020-11-03  Jakub Jelinek  <jakub@redhat.com>

	PR c++/97663
	* parser.c (cp_parser_init_declarator): Don't try to parse
	C++17 deduction guides if there are any type specifiers even when
	type is NULL.

	* g++.dg/cpp1z/class-deduction75.C: New test.

(cherry picked from commit cd41e4a1864c10c7f9141284e82e5cc0a3007806)