This transformation is split into two.  The first transformation solely
takes out the samplers out of structs, and potentially generates array
of array of samplers.  A second transformation is added that takes any
array of array of opaque uniforms and flattens it.

A follow up change will simplify RewriteAtomicCounters which also
handles array of arrays (which is no longer possible), and removes
dependency on shaderStorageBufferArrayDynamicIndexing.

Bug: angleproject:2703
Bug: angleproject:3881
Bug: angleproject:4071
Bug: angleproject:4211
Change-Id: I352bb2bbe65ac49f4d7d753c0ba3160fa3cc925a
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2628138
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>