Only the descriptor binding type is needed. It's an implementation
detail that we currently store the entire original
VkDescriptorSetLayoutBinding structures, which might change in the
future.

This also avoids specializing generated code for things which should
not be treated as pipeline immutable state.

Bug: b/152227757
Change-Id: I20ca7de53e972e77649dec20fcf623f818d4f53c
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/43972
Presubmit-Ready: Nicolas Capens <nicolascapens@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>