[dev.typeparams] cmd/compile/internal/types2: add Config.IgnoreBranches flag

If the new Config.IgnoreBranches flag is set, the typechecker
ignores errors due to misplaced labels, break, continue,
fallthrough, or goto statements.

Since the syntax parser already checks these errors, we need
to disable a 2nd check by the typechecker to avoid duplicate
errors when running the compiler with the new typechecker.

Adjusted test/run.go to not ignore some of the tests that
used to fail because of duplicate errors.

Change-Id: I8756eb1d44f67afef5e57da289cd604b8e1716db
Reviewed-on: https://go-review.googlesource.com/c/go/+/274612


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