[dev.typeparams] cmd/compile/internal/types2: use syntax printer to print expressions

The syntax package has a full-fledged node printer. Use that printer
to create the expression strings needed in error messages, and remove
the local (essentially) duplicate code for creating expression strings.

Change-Id: I03673e5e79b3c1470f8073ebbe840a90fd9053ec
Reviewed-on: https://go-review.googlesource.com/c/go/+/282553


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