Implicit return working for async proc (#20933)

* Implicit return working for asyncdispatch proc

Closes #11558

* Test case

* Test that return value is actually used

* Update tests/async/t11558.nim

Co-authored-by: Andreas Rumpf <rumpf_a@web.de>

tests/async/t11558.nim

@@ -0,0 +1,13 @@
+discard """
+output: "Hello\n9"
+"""
+import std/asyncdispatch
+
+proc foo(): Future[string] {.async.} =
+  "Hello"
+
+proc bar(): Future[int] {.async.} =
+  result = 9
+
+echo waitFor foo()
+echo waitFor bar()