llvm: Ignore error value that is always false

See llvm/llvm-project#121851

For LLVM 20+, this function (`renameModuleForThinLTO`) has no return
value. For prior versions of LLVM, this never failed, but had a
signature which allowed an error value people were handling.

compiler/rustc_codegen_llvm/src/llvm/ffi.rs

@@ -2374,7 +2374,7 @@ unsafe extern "C" {
         Data: &ThinLTOData,
         Module: &Module,
         Target: &TargetMachine,
-    ) -> bool;
+    );
     pub fn LLVMRustPrepareThinLTOResolveWeak(Data: &ThinLTOData, Module: &Module) -> bool;
     pub fn LLVMRustPrepareThinLTOInternalize(Data: &ThinLTOData, Module: &Module) -> bool;
     pub fn LLVMRustPrepareThinLTOImport(