cmd/compile/internal/syntax: handle missing index like in go/parser

Instead of simply reporting an error but otherwise dropping the
index expression from the parse tree when an index is missing
(as in: x[]), create an index expression with a "bad expression"
as index. This matches the behavior of go/parser and permits the
use of the same test case for both parsers.

(It would be simpler to adjust the go/parser to match the syntax
parser's behavior, but that would break backward-compatibility
of the go/parser.)

Adjust the affected test files.

For #54511.

Change-Id: If7668973794604593e869a24b560da92e100b812
Reviewed-on: https://go-review.googlesource.com/c/go/+/424654
Run-TryBot: Robert Griesemer <gri@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>

0 files changed, 0 insertions, 0 deletions