LLVMReactor: #error if usng LLVM 3

An easy to revert step towards https://swiftshader-review.googlesource.com/c/SwiftShader/+/28909.

Bug: b/130343040
Change-Id: I0180e9698bedd2f11749faca1deef141e7000dc4
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/29669
Presubmit-Ready: Ben Clayton <bclayton@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Tested-by: Ben Clayton <bclayton@google.com>
diff --git a/src/Reactor/LLVMReactor.cpp b/src/Reactor/LLVMReactor.cpp
index e2a82a0..a6c6bd3 100644
--- a/src/Reactor/LLVMReactor.cpp
+++ b/src/Reactor/LLVMReactor.cpp
@@ -27,6 +27,8 @@
 #undef max
 
 #if REACTOR_LLVM_VERSION < 7
+#error "LLVM 3 backend is deprecated, and will be removed shortly"
+
 	#include "llvm/Analysis/LoopPass.h"
 	#include "llvm/Constants.h"
 	#include "llvm/Function.h"