[dev.typeparams] go/types: convert testdata/fixedbugs tests to type set sytax

This is a port of CL 324530 to go/types. One error position in
issue39634.go2 was adjusted by a character, to account for go/types'
positioning.

Change-Id: Ie06974ea9ee81d3ae66ef58dba522936ab4ce2d1
Reviewed-on: https://go-review.googlesource.com/c/go/+/326681


Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>