go/parser: parse an ast.IndexExpr for a[]

To be consistent with Go 1.16, and to preserve as much information in
the AST as possible, parse an ast.IndexExpr with BadExpr Index for the
invalid expression a[].

A go/types test had to be adjusted to account for an additional error
resulting from this change.

We don't have a lot of test coverage for parser error recovery, so
rather than write an ad-hoc test for this issue, add a new go/types test
that checks that the indexed operand is used.

Updates #46403

Change-Id: I21e6ff4179746aaa50e530d4091fded450e69824
Reviewed-on: https://go-review.googlesource.com/c/go/+/329791


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