Reactor backend: LLVM

Change-Id: I26387d86e55d9f56589298a1f364ff090ab9f49a
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/49209Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>