Move OutFileName writing into rustc_session

compiler/rustc_session/src/errors.rs

@@ -163,6 +163,13 @@ pub struct FileIsNotWriteable<'a> {
     pub file: &'a std::path::Path,
 }
 
+#[derive(Diagnostic)]
+#[diag(session_file_write_fail)]
+pub(crate) struct FileWriteFail<'a> {
+    pub path: &'a std::path::Path,
+    pub err: String,
+}
+
 #[derive(Diagnostic)]
 #[diag(session_crate_name_does_not_match)]
 pub struct CrateNameDoesNotMatch {