This allows VariableContainer to 'inherit' from one another.
Variables traversed and searched will try the direct VariableContainer, before falling back to searching the extended VariableContainers.

This allows for variable scopes to nest one another.

Bug: b/148401179
Change-Id: I58b317386d74ec54b9c1cc4fa223ef9cc50b160c
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/40532
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Tested-by: Ben Clayton <bclayton@google.com>