The ProgramExecutable class is being created to collect data structures
that are common to both Programs and ProgramPipelines, as well as any
shared functions. This allows callers to request the current
ProgramExecutable from the State and make
Program-/ProgramPipeline-specific queries without needing to know
exactly which responded. This will also allow the necessary data
structures to only be populated and stored within the ProgramExecutable
when necessary and reused as often as necessary.

Bug: angleproject:3570
Change-Id: I101f08ab03421894667b4a426a04d2147489f0e1
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2040512
Commit-Queue: Tim Van Patten <timvp@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>