[ThinLTOBitcodeWriter] Mark pass as required

[aeubanks]

Summary

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

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

 
 public:
   // Writes bitcode to OS. Also write thin link file to ThinLinkOS, if
   // it's not nullptr.
   ThinLTOBitcodeWriterPass(raw_ostream &OS, raw_ostream *ThinLinkOS)
       : OS(OS), ThinLinkOS(ThinLinkOS) {}
 
   PreservedAnalyses run(Module &M, ModuleAnalysisManager &AM);
+
+  static bool isRequired() { return true; }
 };
 
 } // namespace llvm
 
 #endif

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

 ; CHECK-LIMIT-MULTI-PASS: BISECT: NOT running pass (8) PostOrderFunctionAttrsPass on (f4)
 ; CHECK-LIMIT-MULTI-PASS: BISECT: NOT running pass (9) EarlyCSEPass on f4
 
 ; Make sure we don't skip writing the output to stdout.
 ; RUN: opt %s -opt-bisect-limit=0 -passes=early-cse | opt -S | FileCheck %s -check-prefix=CHECK-OUTPUT
 ; 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() {
 entry:
   br label %loop.0
 loop.0:
   br i1 undef, label %loop.0.0, label %loop.1
 loop.0.0: