LLVM changes certain function return types to Expected<T> instead of T.
This change adds an Unwrap that supports both cases, allowing the code
to build in both contexts.

Bug: b/171236524
Change-Id: I461ab440797695ca1e92d4827d6e6d8965dcf6b2
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/49468Tested-by: Antonio Maiorano <amaiorano@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>