gccrs: Handle async keyword for regular implementations

Fixes #2788

gcc/rust/ChangeLog:

	* parse/rust-parse-impl.h (Parser::parse_inherent_impl_item):
	Added switch-case for ASYNC token.

gcc/testsuite/ChangeLog:

	* rust/compile/issue-2788.rs: New test.

Signed-off-by: Kushal Pal <kushalpal109@gmail.com>

gcc/rust/parse/rust-parse-impl.h

@@ -5600,6 +5600,7 @@ Parser<ManagedTokenSource>::parse_inherent_impl_item ()
 	    return nullptr;
 	  }
       }
+    case ASYNC:
     case EXTERN_KW:
     case UNSAFE:
     case FN_KW:

gcc/testsuite/rust/compile/issue-2788.rs

@@ -0,0 +1,10 @@
+// { dg-additional-options "-frust-compile-until=lowering" }
+struct Foo {
+    arg_1: u32,
+    arg_2: i32,
+}
+
+impl Foo {
+    async fn asynchronous_function_1(&self) {}
+    async fn asynchronous_function_2() {}
+}