cmd/compile/internal/syntax: make valid type parameter list in presence of errors

Make sure the parser fills in names and types for type parameter
lists, even in the case of errors.

While at it, adjust some of the test functions to accept generic
code and report all syntax errors.

Added offending source as test for types2.

Fixes #47996.

Change-Id: I449bcf5e2cb80fa2a24cdd3945f484bfca218a06
Reviewed-on: https://go-review.googlesource.com/c/go/+/345476


Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>