Add regression test for rust-lang#60674.

This commit adds a regression test (with current broken behaviour) that
tests that `mut` patterns are not lost when provided as input to a proc macro.

Author: davidtwco

src/test/ui/async-await/auxiliary/issue-60674.rs

@@ -0,0 +1,12 @@
+// force-host
+// no-prefer-dynamic
+#![crate_type = "proc-macro"]
+
+extern crate proc_macro;
+use proc_macro::TokenStream;
+
+#[proc_macro_attribute]
+pub fn attr(_args: TokenStream, input: TokenStream) -> TokenStream {
+    println!("{}", input);
+    TokenStream::new()
+}

src/test/ui/async-await/issue-60674.rs

@@ -0,0 +1,14 @@
+// aux-build:issue-60674.rs
+// compile-pass
+// edition:2018
+#![feature(async_await)]
+
+// This is a regression test that ensures that `mut` patterns are not lost when provided as input
+// to a proc macro.
+
+extern crate issue_60674;
+
+#[issue_60674::attr]
+async fn f(mut x: u8) {}
+
+fn main() {}

src/test/ui/async-await/issue-60674.stdout

@@ -0,0 +1 @@
+async fn f(x: u8) { }