[clang][fatlto] Don't set ThinLTO module flag with FatLTO (#75079)

Since FatLTO now uses the UnifiedLTO pipeline, we should not set the
ThinLTO module flag to true, since it may cause an assertion failure.
See #70703 for context.

Author: ilovepi

clang/lib/CodeGen/BackendUtil.cpp

@@ -1068,11 +1068,8 @@ void EmitAssemblyHelper::RunOptimizationPipeline(
     }
   }
   if (CodeGenOpts.FatLTO) {
-    // Set module flags, like EnableSplitLTOUnit and UnifiedLTO, since FatLTO
+    // Set the EnableSplitLTOUnit and UnifiedLTO module flags, since FatLTO
     // uses a different action than Backend_EmitBC or Backend_EmitLL.
-    if (!TheModule->getModuleFlag("ThinLTO"))
-      TheModule->addModuleFlag(llvm::Module::Error, "ThinLTO",
-                               uint32_t(CodeGenOpts.PrepareForThinLTO));
     if (!TheModule->getModuleFlag("EnableSplitLTOUnit"))
       TheModule->addModuleFlag(llvm::Module::Error, "EnableSplitLTOUnit",
                                uint32_t(CodeGenOpts.EnableSplitLTOUnit));

clang/test/CodeGen/fat-lto-objects.c

@@ -35,8 +35,7 @@
 //   SPLIT: ![[#]] = !{i32 1, !"EnableSplitLTOUnit", i32 1}
 // NOSPLIT: ![[#]] = !{i32 1, !"EnableSplitLTOUnit", i32 0}
 
-/// Check that the ThinLTO metadata is set true for both full and thin LTO, since FatLTO is based on UnifiedLTO.
-//     FULL: ![[#]] = !{i32 1, !"ThinLTO", i32 1}
+// FULL-NOT: ![[#]] = !{i32 1, !"ThinLTO", i32 0}
 // THIN-NOT: ![[#]] = !{i32 1, !"ThinLTO", i32 0}
 
 /// FatLTO always uses UnifiedLTO. It's an error if they aren't set together