go/parser, go/printer: fix parsing of ambiguous type parameter lists

This is a port of CL 370774 to go/parser and go/printer. It is adjusted
for the slightly different factoring of parameter list parsing and
printing in go/parser and go/printer.

For #49482

Change-Id: I1c5b1facddbfcb7f7b2be356c817fc7e608223f1
Reviewed-on: https://go-review.googlesource.com/c/go/+/385575


Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>