go/printer: fix printing for possibly ambiguous type constraints
This is a port of the printer changes from CLs 402256 and 404397
in the syntax package to go/printer, with adjustments for the
different AST structure and test framework.
For #52559.
Change-Id: Ib7165979a4bd9df91f7f0f1c23b756a41ca31eb3
Reviewed-on: https://go-review.googlesource.com/c/go/+/404194 Reviewed-by: Ian Lance Taylor <iant@google.com> Reviewed-by: Robert Findley <rfindley@google.com>