The member ProgramState::mSamplerBindings is being moved to
ProgramExecutable to allow ProgramExecutable::getSamplerBindings() to
answer the query without relying on the Program[Pipeline]State.

Bug: angleproject:4520
Test: Build/CQ
Change-Id: I0daa997424d6e2aa5172e0731da221db72063435
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2233363Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Courtney Goeltzenleuchter <courtneygo@google.com>
Commit-Queue: Tim Van Patten <timvp@google.com>