GenericValue is only used by SpirvShader so move it as a private inner class of SpirvShader.

Part of the foundations of much larger refactoring required for function inlining in SpirvShader.

Bug: b/133213304
Change-Id: I75484c3d036b82f3bc6a735034a7c1b04f05f600
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/33350Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Tested-by: Ben Clayton <bclayton@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>