go/types: print "nil" rather than "untyped nil"

go/types: print "nil" rather than "untyped nil"

This is a port of CL 366276 from types2 to go/types
with minor adjustments due to the slightly different
handling of nil in go/types.

It uses some more detailed error strings in stmt0.src;
the same changes are made to the corresponding types2
file.

For #48852.

Change-Id: I2cdf258799bcbe2d12bbadaf67b8b4504b356bd0
Reviewed-on: https://go-review.googlesource.com/c/go/+/366277
Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>