Fix diagnostic and add a test for it

compiler/rustc_resolve/src/errors.rs

@@ -1089,8 +1089,8 @@ pub(crate) struct ToolWasAlreadyRegistered {
 #[derive(Diagnostic)]
 #[diag(resolve_tool_only_accepts_identifiers)]
 pub(crate) struct ToolOnlyAcceptsIdentifiers {
-    #[label]
     #[primary_span]
+    #[label]
     pub(crate) span: Span,
     pub(crate) tool: Symbol,
 }

tests/ui/tool-attributes/invalid-tool.rs

@@ -0,0 +1,6 @@
+#![feature(register_tool)]
+
+#![register_tool(1)]
+//~^ ERROR `register_tool` only accepts identifiers
+
+fn main() {}

tests/ui/tool-attributes/invalid-tool.stderr

@@ -0,0 +1,8 @@
+error: `register_tool` only accepts identifiers
+  --> $DIR/invalid-tool.rs:3:18
+   |
+LL | #![register_tool(1)]
+   |                  ^ not an identifier
+
+error: aborting due to 1 previous error
+