Vulkan doesn't treat atomic counters especially, and they are emulated
with atomic access to storage buffers.

A single atomic counter buffer binding per pipeline is supported.  All
the atomic counters identify an offset within this buffer.  The shader
is modified to include a storage buffer definition with
`uint counters[];` as the only field.

A compiler pass replaces atomic counter definitions with variables that
hold the corresponding offset parameter, as well as changing atomic_uint
types to just uint (as the offset).  Where an atomic counter variable is
used, it is replaced with the offset variable (plus the array index, if
array).  At the same time, built-in `atomicCounter*` functions are
replaced with a corresponding `atomic*` function and
`memoryBarrierAtomicCounter` is replaced with `memoryBarrierBuffer`.

Bug: angleproject:3566
Change-Id: Iefb3d47de6a5cb3072bfa0cb94a46ac6a886d369
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1704635
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>