[ThinLTOBitcodeWriter] Mark pass as required

Or else with -opt-bisect-limit we don't write ThinLTO bitcode.

Reviewed By: asbirlea

Differential Revision: https://reviews.llvm.org/D133378

llvm/include/llvm/Transforms/IPO/ThinLTOBitcodeWriter.h

@@ -34,6 +34,8 @@ public:
       : OS(OS), ThinLinkOS(ThinLinkOS) {}
 
   PreservedAnalyses run(Module &M, ModuleAnalysisManager &AM);
+
+  static bool isRequired() { return true; }
 };
 
 } // namespace llvm

llvm/test/Other/opt-bisect-new-pass-manager.ll

@@ -99,6 +99,10 @@
 ; RUN: opt %s -opt-bisect-limit=0 -passes=early-cse -S | FileCheck %s -check-prefix=CHECK-OUTPUT
 ; CHECK-OUTPUT: define void @f1
 
+; Make sure we write ThinLTO bitcode
+; RUN: opt %s -opt-bisect-limit=0 -disable-verify -thinlto-bc -o /dev/null 2>&1 | FileCheck --allow-empty %s -check-prefix=CHECK-THINLTO
+; CHECK-THINLTO-NOT: NOT running pass
+
 declare i32 @g()
 
 define void @f1() {