Invoke OptimizerLastEPCallbacks in PreLinkThinLTO

The default ThinLTO pre-link pipeline does not include optimizer last
extension points. Thus, when using the new LLVM pass manager & ThinLTO
& sanitizers on any opt-level different from zero, the sanitizer
function passes would be omitted from the pipeline.

Add optimizer last extensions points manually to the pipeline, but guard
registration with stage check in the case this behaviour changes in the
future.

src/rustllvm/PassWrapper.cpp

@@ -868,15 +868,23 @@ LLVMRustOptimizeWithNewPassManager(
     } else {
       for (const auto &C : PipelineStartEPCallbacks)
         PB.registerPipelineStartEPCallback(C);
-      for (const auto &C : OptimizerLastEPCallbacks)
-        PB.registerOptimizerLastEPCallback(C);
+      if (OptStage != LLVMRustOptStage::PreLinkThinLTO) {
+        for (const auto &C : OptimizerLastEPCallbacks)
+          PB.registerOptimizerLastEPCallback(C);
+      }
 
       switch (OptStage) {
       case LLVMRustOptStage::PreLinkNoLTO:
         MPM = PB.buildPerModuleDefaultPipeline(OptLevel, DebugPassManager);
         break;
       case LLVMRustOptStage::PreLinkThinLTO:
         MPM = PB.buildThinLTOPreLinkDefaultPipeline(OptLevel, DebugPassManager);
+        if (!OptimizerLastEPCallbacks.empty()) {
+          FunctionPassManager FPM(DebugPassManager);
+          for (const auto &C : OptimizerLastEPCallbacks)
+            C(FPM, OptLevel);
+          MPM.addPass(createModuleToFunctionPassAdaptor(std::move(FPM)));
+        }
         break;
       case LLVMRustOptStage::PreLinkFatLTO:
         MPM = PB.buildLTOPreLinkDefaultPipeline(OptLevel, DebugPassManager);