Fix void async in templates (#17562)

* Fix https://github.com/nim-lang/Nim/issues/16159
* Add test for async in template
* Shorten the expression
* Even shorter

Co-authored-by: Clyybber <darkmine956@gmail.com>

tests/async/tasyncintemplate.nim

@@ -0,0 +1,12 @@
+discard """
+  output: 42
+"""
+
+import asyncdispatch
+
+template foo() =
+  proc temp(): Future[int] {.async.} = return 42
+  proc tempVoid(): Future[void] {.async.} = echo await temp()
+
+foo()
+waitFor tempVoid()