Rollup merge of rust-lang#130477 - tmandry:revert-llvm-20-lto, r=tmandry

Revert rust-lang#129749 to fix segfault in LLVM

This reverts commit 8c7a7e3, reversing changes made to a00bd75.

Reported in rust-lang#129749 (comment). `@nikic's` theory is that the LLVM API changed in a way that makes it impossible to use concurrently from multiple threads (llvm/llvm-project#106427 (comment)). I pinged `@krasimirgg` who was fine with reverting.

r? `@rust-lang/wg-llvm`

compiler/rustc_llvm/llvm-wrapper/PassWrapper.cpp

@@ -1211,11 +1211,7 @@ struct LLVMRustThinLTOData {
   // Not 100% sure what these are, but they impact what's internalized and
   // what's inlined across modules, I believe.
 #if LLVM_VERSION_GE(18, 0)
-#if LLVM_VERSION_GE(20, 0)
-  FunctionImporter::ImportListsTy ImportLists;
-#else
   DenseMap<StringRef, FunctionImporter::ImportMapTy> ImportLists;
-#endif
   DenseMap<StringRef, FunctionImporter::ExportSetTy> ExportLists;
   DenseMap<StringRef, GVSummaryMapTy> ModuleToDefinedGVSummaries;
 #else
@@ -1424,13 +1420,13 @@ LLVMRustPrepareThinLTOInternalize(const LLVMRustThinLTOData *Data,
   return true;
 }
 
-extern "C" bool LLVMRustPrepareThinLTOImport(LLVMRustThinLTOData *Data,
+extern "C" bool LLVMRustPrepareThinLTOImport(const LLVMRustThinLTOData *Data,
                                              LLVMModuleRef M,
                                              LLVMTargetMachineRef TM) {
   Module &Mod = *unwrap(M);
   TargetMachine &Target = *unwrap(TM);
 
-  const auto &ImportList = Data->ImportLists[Mod.getModuleIdentifier()];
+  const auto &ImportList = Data->ImportLists.lookup(Mod.getModuleIdentifier());
   auto Loader = [&](StringRef Identifier) {
     const auto &Memory = Data->ModuleMap.lookup(Identifier);
     auto &Context = Mod.getContext();
@@ -1613,7 +1609,7 @@ extern "C" void LLVMRustComputeLTOCacheKey(RustStringRef KeyOut,
                                            LLVMRustThinLTOData *Data) {
   SmallString<40> Key;
   llvm::lto::Config conf;
-  const auto &ImportList = Data->ImportLists[ModId];
+  const auto &ImportList = Data->ImportLists.lookup(ModId);
   const auto &ExportList = Data->ExportLists.lookup(ModId);
   const auto &ResolvedODR = Data->ResolvedODR.lookup(ModId);
   const auto &DefinedGlobals = Data->ModuleToDefinedGVSummaries.lookup(ModId);