[spirv] Cleanup the SPIR-V writer options.

This CL splits apart the monolithic SPIR-V writer options and adds two
new structs.

The first, `Workarounds` contains any flags for polyfills we emit based
on specific drivers or conditions. These flags are all related to
working around issues in drivers.

The second, `Extensions` contains any flags which enable or disable the
use of vulkan extensions and capabilities. These flags are all related
to filling in for missing extensions or using extensions when they're
available.

Change-Id: I11529e3efeb2b0653f7d5f846f617b0b86cb839d
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/276915
Reviewed-by: Peter McNeeley <petermcneeley@google.com>
Commit-Queue: dan sinclair <dsinclair@chromium.org>
Reviewed-by: James Price <jrprice@google.com>