Treat mid-render-pass glClear* commands the same as glDraw*
commands, generating a hierarchy.  This results in
vkCmdClearAttachment commands being nested under glClear, instead of
being a peer of glDraw* commands.

Bug: b/183547523
Change-Id: Ibc6900b0485fd174d79c8fe6c94ea17dbefa520b
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2941364Reviewed-by: Mark Lobodzinski <mark@lunarg.com>
Reviewed-by: Mike Schuchardt <mikes@lunarg.com>
Reviewed-by: Ian Elliott <ianelliott@google.com>
Commit-Queue: Ian Elliott <ianelliott@google.com>