Rollup merge of #54977 - estebank:macro-arg-parse, r=pnkfelix

Accept `Option<Box<$t:ty>>` in macro argument

Given the following code, compile successfuly:

```
macro_rules! test {
    (
        fn fun() -> Option<Box<$t:ty>>;
    ) => {
        fn fun(x: $t) -> Option<Box<$t>>
        { Some(Box::new(x)) }
    }
}

test! {
    fn fun() -> Option<Box<i32>>;
}
```

Fix #25274.

src/test/run-pass/macros/issue-25274.rs

@@ -0,0 +1,16 @@
+macro_rules! test {
+    (
+        fn fun() -> Option<Box<$t:ty>>;
+    ) => {
+        fn fun(x: $t) -> Option<Box<$t>>
+        { Some(Box::new(x)) }
+    }
+}
+
+test! {
+    fn fun() -> Option<Box<i32>>;
+}
+
+fn main() {
+    println!("{}", fun(0).unwrap());
+}