This will enable us to lazily create the underlying Value* in subsequent
changes.

Also make Argument<T> encapsulate its value, and make it type-safe.

Bug: b/155302798
Change-Id: I50eb16be06bc83e6c87b11f11effed33bee7107a
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/44671Reviewed-by: Antonio Maiorano <amaiorano@google.com>
Presubmit-Ready: Nicolas Capens <nicolascapens@google.com>
Kokoro-Result: kokoro <noreply+kokoro@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>