Add LLVM module flags required for the VFE opt

To apply the optimization the `Virtual Function Elim` module flag has to
be set. To apply this optimization post-link the `LTOPostLink` module
flag has to be set.

compiler/rustc_codegen_llvm/src/context.rs

@@ -326,6 +326,15 @@ pub unsafe fn create_module<'ll>(
         )
     }
 
+    if sess.opts.debugging_opts.virtual_function_elimination {
+        llvm::LLVMRustAddModuleFlag(
+            llmod,
+            llvm::LLVMModFlagBehavior::Error,
+            "Virtual Function Elim\0".as_ptr().cast(),
+            1,
+        );
+    }
+
     llmod
 }