allow non-monomorphize modules to access hard-coded error message through new struct, use fluent message in monomorphize

compiler/rustc_hir/src/errors.rs

@@ -0,0 +1,9 @@
+use crate::LangItem;
+
+pub struct LangItemError(pub LangItem);
+
+impl ToString for LangItemError {
+    fn to_string(&self) -> String {
+        format!("requires `{}` lang_item", self.0.name())
+    }
+}