Run ObjCContractPass in Distributed Thin-LTO Pipeline

Prior to this patch, when using -fthinlto-index= the
ObjCARCContractPass isn't run prior to CodeGen, and
instruction selection fails on IR containing
arc intrinstics.

Author: Nuri Amari

clang/lib/CodeGen/BackendUtil.cpp

@@ -1289,6 +1289,9 @@ static void runThinLTOBackend(
     };
     break;
   default:
+    Conf.PreCodeGenPassesHook = [](legacy::PassManager &Pm) {
+      Pm.add(createObjCARCContractPass());
+    };
     Conf.CGFileType = getCodeGenFileType(Action);
     break;
   }

clang/test/CodeGen/thinlto-distributed-objc-contract-pass.ll

@@ -0,0 +1,19 @@
+; RUN: opt -thinlto-bc -o %t.o %s
+
+; RUN: llvm-lto2 run -thinlto-distributed-indexes %t.o \
+; RUN:   -o %t2.index \
+; RUN:   -r=%t.o,_use_arc,px
+
+; RUN: %clang_cc1 -triple x86_64-apple-darwin \
+; RUN:   -emit-obj -fthinlto-index=%t.o.thinlto.bc \
+; RUN:   -o %t.native.o -x ir %t.o
+
+target datalayout = "e-m:o-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-apple-darwin"
+
+define void @use_arc(ptr %a, ptr %b) {
+  call void (...) @llvm.objc.clang.arc.use(i8* %a, i8* %b) nounwind
+  ret void
+}
+
+declare void @llvm.objc.clang.arc.use(...) nounwind