Adding a version map entry for 10.15.6 to default to XCode 12
GM (12a7209) as machines upgrade to 10.15.

Bug: 1128042, 1128044
Change-Id: I6396b67fdce006d19be35d972602bac3bd4ae0a7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2422449Reviewed-by: Dirk Pranke <dpranke@google.com>
Commit-Queue: Jeff Yoon <jeffyoon@chromium.org>