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

A few things are being moved into this class:
- mVariableInfoMap
This information will be required to defer translating the SPIR-V
until when the vulkan pipeline layout is determined and the actual
locations are known.   This will also allow removing determining
these locations twice (during GLSL->SPIR-V compilation and pipeline
layout).
- createPipelineLayout()
- update*DescriptorSet()
- Descriptor Set Layout lists
- Pipeline Layout
- Various other functions/members related to pipeplines, descriptor
sets, etc.

Bug: angleproject:3570
Change-Id: I4b5ababeafec865148783c8ffd4c15f659f4856d
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2055656
Commit-Queue: Tim Van Patten <timvp@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>