Don't begin defining a function when codegening a naked function

While LLVM is rather permissive in this regards, some other codegen
backends demand that once you declare a function for definition you
actually define contents of the function, which doesn't happen for naked
functions as we actually generate assembly for them.

Author: bjorn3

compiler/rustc_codegen_ssa/src/mir/mod.rs

@@ -170,19 +170,19 @@ pub fn codegen_mir<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
 ) {
     assert!(!instance.args.has_infer());
 
+    let mut mir = cx.tcx().instance_mir(instance.def);
+
+    if cx.tcx().codegen_fn_attrs(instance.def_id()).flags.contains(CodegenFnAttrFlags::NAKED) {
+        crate::mir::naked_asm::codegen_naked_asm::<Bx>(cx, &mir, instance);
+        return;
+    }
+
     let tcx = cx.tcx();
     let llfn = cx.get_fn(instance);
 
-    let mut mir = tcx.instance_mir(instance.def);
-
     let fn_abi = cx.fn_abi_of_instance(instance, ty::List::empty());
     debug!("fn_abi: {:?}", fn_abi);
 
-    if tcx.codegen_fn_attrs(instance.def_id()).flags.contains(CodegenFnAttrFlags::NAKED) {
-        crate::mir::naked_asm::codegen_naked_asm::<Bx>(cx, &mir, instance);
-        return;
-    }
-
     if tcx.features().ergonomic_clones() {
         let monomorphized_mir = instance.instantiate_mir_and_normalize_erasing_regions(
             tcx,