rustc_codegen_llvm: move DISubprogram creation to a dbg_scope_fn method.

compiler/rustc_codegen_llvm/src/common.rs

@@ -80,6 +80,7 @@ impl Funclet<'ll> {
 
 impl BackendTypes for CodegenCx<'ll, 'tcx> {
     type Value = &'ll Value;
+    // FIXME(eddyb) replace this with a `Function` "subclass" of `Value`.
     type Function = &'ll Value;
 
     type BasicBlock = &'ll BasicBlock;