clear in-scope lifetimes for nested items in HIR lowering

This was causing us to incorrectly think the lifetimes were
already declared on the scope for the nested item, when in fact
they are not inherited.

src/test/ui/async-await/nested-in-impl.rs

@@ -0,0 +1,17 @@
+// Test that async fn works when nested inside of
+// impls with lifetime parameters.
+//
+// check-pass
+// edition:2018
+
+#![feature(async_await)]
+
+struct Foo<'a>(&'a ());
+
+impl<'a> Foo<'a> {
+    fn test() {
+        async fn test() {}
+    }
+}
+
+fn main() { }