go/types: add -halt flag to ease debugging in test mode

Specifying -halt in `go test -run Check$ -halt` causes a panic
upon encountering the first error. The stack trace is useful to
determine what code path issued the error.

Change-Id: I2e17e0014ba87505b01786980b98565f468065bf
Reviewed-on: https://go-review.googlesource.com/c/go/+/190257


Reviewed-by: Matthew Dempsky <mdempsky@google.com>