AddDate now retrieves location from t.Location() to ensure that
it never calls Date with a nil location.

Added test for this bug on all Time's methods

Fixes #15852

Change-Id: Id2a222af56993f741ad0b802a2c3b89e8e463926
Reviewed-on: https://go-review.googlesource.com/23561Reviewed-by: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>