In this testcase, in C++17 mode after parsing the template-id fails,
we try to treat the template-name as a class template deduction
placeholder, producing an auto type, and then we try to take its
TREE_TYPE when calling cp_parser_check_for_invalid_type_id, which
doesn't work because it's a type, not a TYPE_DECL.  Fixed by removing
the TREE_TYPE and adjusting cp_parser_check_for_invalid_type_id to
handle TYPE_DECL.

Really we shouldn't be creating a deduction placeholder in the first
place when the template name is followed by <, but that distinction
shouldn't affect well-formed code since we try to parse a template-id
first.

I still want to improve our handling of parse errors in template-ids;
we never actually diagnose the syntax error here because it's
swallowed by tentative parsing, we just say "argument 1 is invalid".

Tested x86_64-pc-linux-gnu, applying to trunk.