The parser CheckBranches mode checked correct use of break, continue,
and labels, but not of fallthrough statements.

This CL adds checking of fallthrough statements as well.

For #51456.

Change-Id: I5000388011973724f80c59a6aaf015e3bb70faea
Reviewed-on: https://go-review.googlesource.com/c/go/+/414134
Reviewed-by: Robert Griesemer <gri@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>

The parser CheckBranches mode checked correct use of break, continue,
and labels, but not of fallthrough statements.

This CL adds checking of fallthrough statements as well.

For golang#51456.

Change-Id: I5000388011973724f80c59a6aaf015e3bb70faea
Reviewed-on: https://go-review.googlesource.com/c/go/+/414134
Reviewed-by: Robert Griesemer <gri@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>

The previous change implemented the missing fallthrough checking
in the parser. Therefore we can now disable the duplicate check
in the type checker:

- rename (types2.Config.)IngoreLabels to IgnoreBranches to more
  accurately reflect its functionality

- now also ignore break/continue/fallthroughs, not just labels

The IgnoreBranches flag only exists for types2, for use with
the compiler. There's no need to port this code to go/types.

Note: An alternative (and perhaps better) approach would be
to not use the the parser's CheckBranches mode and instead
enable (i.e. not disable) the branch/label checking in the
type checker. However, this requires a bit more work because
the type checker's error messages about goto's jumping over
variables don't have access to the variable names, which are
desired in the error messages.

Fixes golang#51456.

Change-Id: Ib2e71e811d4e84e4895b729646e879fd43b12dcd
Reviewed-on: https://go-review.googlesource.com/c/go/+/414135
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>