`vk::dbg::Reference` is a `vk::dbg::Value` that reads and writes from a value stored in memory.

Used for displaying variables.

Bug: b/145351270
Change-Id: I64146cb11b13267bef15d727af82a2256a9f25ce
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/39881Reviewed-by: Antonio Maiorano <amaiorano@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Tested-by: Ben Clayton <bclayton@google.com>
Presubmit-Ready: Ben Clayton <bclayton@google.com>