Warn on inert attributes used on bang macro invocation

These attributes are currently discarded.
This may change in the future (see #63221), but for now,
placing inert attributes on a macro invocation does nothing,
so we should warn users about it.

Technically, it's possible for there to be attribute macro
on the same macro invocation (or at a higher scope), which
inspects the inert attribute. For example:

```rust
#[look_for_inline_attr]
#[inline]
my_macro!()

#[look_for_nested_inline]
mod foo { #[inline] my_macro!() }
```

However, this would be a very strange thing to do.
Anyone running into this can manually suppress the warning.

src/test/ui/lint/inert-attr-macro.rs

@@ -0,0 +1,20 @@
+// check-pass
+
+#![warn(unused)]
+
+macro_rules! foo {
+    () => {}
+}
+
+fn main() {
+    #[inline] foo!(); //~ WARN unused attribute `inline`
+
+    // This does nothing, since `#[allow(warnings)]` is itself
+    // an inert attribute on a macro call
+    #[allow(warnings)] #[inline] foo!(); //~ WARN unused attribute `allow`
+    //~^ WARN unused attribute `inline`
+
+    // This does work, since the attribute is on a parent
+    // of the macro invocation.
+    #[allow(warnings)] { #[inline] foo!(); }
+}

src/test/ui/lint/inert-attr-macro.stderr

@@ -0,0 +1,44 @@
+warning: unused attribute `inline`
+  --> $DIR/inert-attr-macro.rs:10:5
+   |
+LL |     #[inline] foo!();
+   |     ^^^^^^^^^
+   |
+note: the lint level is defined here
+  --> $DIR/inert-attr-macro.rs:3:9
+   |
+LL | #![warn(unused)]
+   |         ^^^^^^
+   = note: `#[warn(unused_attributes)]` implied by `#[warn(unused)]`
+note: the built-in attribute `inline` will be ignored, since it's applied to the macro invocation `foo`
+  --> $DIR/inert-attr-macro.rs:10:15
+   |
+LL |     #[inline] foo!();
+   |               ^^^
+
+warning: unused attribute `allow`
+  --> $DIR/inert-attr-macro.rs:14:5
+   |
+LL |     #[allow(warnings)] #[inline] foo!();
+   |     ^^^^^^^^^^^^^^^^^^
+   |
+note: the built-in attribute `allow` will be ignored, since it's applied to the macro invocation `foo`
+  --> $DIR/inert-attr-macro.rs:14:34
+   |
+LL |     #[allow(warnings)] #[inline] foo!();
+   |                                  ^^^
+
+warning: unused attribute `inline`
+  --> $DIR/inert-attr-macro.rs:14:24
+   |
+LL |     #[allow(warnings)] #[inline] foo!();
+   |                        ^^^^^^^^^
+   |
+note: the built-in attribute `inline` will be ignored, since it's applied to the macro invocation `foo`
+  --> $DIR/inert-attr-macro.rs:14:34
+   |
+LL |     #[allow(warnings)] #[inline] foo!();
+   |                                  ^^^
+
+warning: 3 warnings emitted
+