The storage buffers are placed in the same descriptor set as uniform
buffers.  Some refactoring is done to reuse code that handles UBOs to
handle SSBOs as well.  A good number of tests still fail as they test
SSBOs in conjunction with compute shaders.

Bug: angleproject:3561
Change-Id: Ia33c1f68e6f6402c746f5919ede87b2c308cf81c
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1687126
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>