Update StateCache::updateActiveShaderStorageBufferIndices() and
StateCache::updateActiveImageUnitIndices to get the storage buffers and
image bindings from the ProgramExecutable, respectively. This requires
updating the ProgramPipeline's ProgramExecutable to build up its vector
of buffers/images from each Program's ProgramExecutable.

Bug: angleproject:4869
Test: VertexAttributeTestES31.UsePpoComputeShaderToUpdateVertexBuffer
Test: SimpleStateChangeTestES31.InvalidateThenStorageWriteThenBlendPpo
Change-Id: I68b7d23eedda910c3dcbf5f9c50b74b5e80134d8
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2327701
Commit-Queue: Tim Van Patten <timvp@google.com>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Charlie Lao <cclao@google.com>