kClearValueDepth/StencilIndex is renamed and repurposed in other places
where depth and stencil are placed at indices MAX_DRAW_BUFFERS and
MAX_DRAW_BUFFERS+1.

Bug: angleproject:4836
Change-Id: Idaeff5017d944d786a5f388c4f1ce3a4e3fe9b7d
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2437505
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>