c++: diagnostic for template placeholder in parm [PR106793]

c++: diagnostic for template placeholder in parm [PR106793]

Talking about the declarator form doesn't help when fixing that would get
you a different error about placeholders not being valid in a parameter.

This also adds a <> fixit, which isn't enough for most templates, but is a
start.

	PR c++/106793

gcc/cp/ChangeLog:

	* decl.cc (grokdeclarator): Improve placeholder diagnostics.
	* parser.cc (cp_parser_type_id_1): Add fixit.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp23/auto-array2.C: Adjust.
	* g++.dg/cpp1z/class-deduction113.C: New test.