Schedule ThinLTOBuffer passes again after sanitizer passes

This works around a design defect in the LLVM 12 pass builder
implementation. In LLVM 13, the PreLink ThinLTO pipeline properly
respects the OptimizerLastEPCallbacks.

Author: nikic

compiler/rustc_llvm/llvm-wrapper/PassWrapper.cpp

@@ -1015,7 +1015,12 @@ LLVMRustOptimizeWithNewPassManager(
       case LLVMRustOptStage::PreLinkThinLTO:
 #if LLVM_VERSION_GE(12, 0)
         MPM = PB.buildThinLTOPreLinkDefaultPipeline(OptLevel);
-        NeedThinLTOBufferPasses = false;
+        // The ThinLTOPreLink pipeline already includes ThinLTOBuffer passes. However, callback
+        // passes may still run afterwards. This means we need to run the buffer passes again.
+        // FIXME: In LLVM 13, the ThinLTOPreLink pipeline also runs OptimizerLastEPCallbacks
+        // before the RequiredLTOPreLinkPasses, in which case we can remove these hacks.
+        if (OptimizerLastEPCallbacks.empty())
+          NeedThinLTOBufferPasses = false;
 #else
         MPM = PB.buildThinLTOPreLinkDefaultPipeline(OptLevel, DebugPassManager);
 #endif