The Array<> assignment operator gets auto-generated, but is unsafe as it
overwrites the Variable address field.

Also delete the Variable assignment operator.

Bug b/129356087

Change-Id: If6d5945f2a56f9a81bbc1491a524e3f17c1561da
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/28088
Presubmit-Ready: Nicolas Capens <nicolascapens@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>