Add regression test for rust-lang#99647

Signed-off-by: Yuki Okushi <jtitor@2k36.org>

src/test/ui/const-generics/generic_const_exprs/issue-99647.rs

@@ -0,0 +1,15 @@
+// edition:2018
+// run-pass
+
+#![allow(incomplete_features)]
+#![feature(generic_const_exprs)]
+
+#[allow(unused)]
+async fn foo<'a>() {
+    let _data = &mut [0u8; { 1 + 4 }];
+    bar().await
+}
+
+async fn bar() {}
+
+fn main() {}