Use of the member variable mProgramState is being removed to reduce the
circular coupling of ProgramExecutable and ProgramState.

Bug: angleproject:4520
Test: Build/CQ
Change-Id: I31d7e607fb06a374a0b0e1d35eb2a092614badf5
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2243323
Commit-Queue: Tim Van Patten <timvp@google.com>
Reviewed-by: Courtney Goeltzenleuchter <courtneygo@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>