- Provides a point to complain if something is not yet defined.
- Allows avoiding ugly syntax from use of unique_ptr

Bug: b/124388146
Change-Id: Ib55023fff5c90b71a41c5f558544be65250bb67d
Reviewed-on: https://swiftshader-review.googlesource.com/c/24791Tested-by: Chris Forbes <chrisforbes@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>