Reactor backend: LLVM

Change-Id: Iccd8573f30562e5aaf0f5f56a389d6c1bf5da30e
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/55068
Kokoro-Result: kokoro <noreply+kokoro@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Tested-by: Alexis Hétu <sugoi@google.com>