c++: Fix -Wvexing-parse ICE with omitted int [PR97762]

c++: Fix -Wvexing-parse ICE with omitted int [PR97762]

For declarations like

  long f();

decl_specifiers->type will be NULL, but I neglected to handle this case,
therefore we ICE.  So handle this case by pretending we've seen 'int',
which is good enough for -Wvexing-parse's purposes.

gcc/cp/ChangeLog:

	PR c++/97762
	* parser.c (warn_about_ambiguous_parse): Handle the case when
	there is no type in the decl-specifiers.

gcc/testsuite/ChangeLog:

	PR c++/97762
	* g++.dg/warn/Wvexing-parse8.C: New test.