Instead of declaring a bunch of private forward declarations in SpirvShader, create a single Impl struct, which can hold all the forward declarations. The names of these forward declarations can then directly match the SpirvShader file name suffixes.

The Impl struct can also be used later to store pointers to any internal state.

This was proposed here: https://swiftshader-review.googlesource.com/c/SwiftShader/+/38929/1/src/Pipeline/SpirvShaderGroup.cpp#23

Bug: b/145351270
Change-Id: Ie0de5e651fce9969fe0c24f51ae57652cade54d1
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/39883Reviewed-by: Antonio Maiorano <amaiorano@google.com>
Tested-by: Ben Clayton <bclayton@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>