go/parser, go/ast: correctly take into account presence of } in block

Correctly track whether the closing } of a block (or a function body)
is present or not in the AST and report correct End() positions in
each case.

There are more cases like this but this CL addresses an immediate
issue and sets a precedent for how to fix similar cases if a need
arises.

Fixes #33649.

Change-Id: Id6662ddaac09f3c15f8003edc9275fe2b0c41c78
Reviewed-on: https://go-review.googlesource.com/c/go/+/202581


Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Alan Donovan <adonovan@google.com>