Set 'PrepareForThinLTO' whenever doing cross-language LTO.

Author: michaelwoerister

src/librustc_codegen_llvm/back/write.rs

@@ -546,7 +546,9 @@ unsafe fn optimize(cgcx: &CodegenContext,
                 llvm::LLVMRustAddAnalysisPasses(tm, fpm, llmod);
                 llvm::LLVMRustAddAnalysisPasses(tm, mpm, llmod);
                 let opt_level = config.opt_level.unwrap_or(llvm::CodeGenOptLevel::None);
-                let prepare_for_thin_lto = cgcx.lto == Lto::Thin || cgcx.lto == Lto::ThinLocal;
+                let prepare_for_thin_lto = cgcx.lto == Lto::Thin ||
+                                           cgcx.lto == Lto::ThinLocal ||
+                    (cgcx.lto != Lto::Fat && cgcx.opts.debugging_opts.cross_lang_lto.enabled());
                 with_llvm_pmb(llmod, &config, opt_level, prepare_for_thin_lto, &mut |b| {
                     llvm::LLVMPassManagerBuilderPopulateFunctionPassManager(b, fpm);
                     llvm::LLVMPassManagerBuilderPopulateModulePassManager(b, mpm);