go/types: match types2 string when printing composite literals
Given a composite literal type S, rather than always printing
(S literal) for a composite literals, print S{} if the literal
has no elements, and print S{…} as a short form (suitable for
error messages) if there are elements. This matches types2 and
also Go1.17 compiler behavior (except that the original compiler
would print ... rather than …). Using … rather than ... makes
it clearer that we don't have real Go syntax, and it's also more
compact.
For #54511.
Change-Id: I5991e8060232f16ecbf4a1fe4ae091598fc76b68
Reviewed-on: https://go-review.googlesource.com/c/go/+/425006 Reviewed-by: Robert Griesemer <gri@google.com> Reviewed-by: Alan Donovan <adonovan@google.com>