[Macros] Tighten restriction on non-expression macros not having return types

There is no reason to special-case `Void` to permit it. Rather, make
this a syntactic rule. Thanks to Alex Hoppen for the suggestion.

test/Macros/parsing.swift

@@ -51,7 +51,7 @@ macro am1()
   named, // expected-error{{introduced name kind 'named' requires a single argument '(name)'}}
   arbitrary(a) // expected-error{{introduced name kind 'arbitrary' must not have an argument}}
 )
-macro am2() -> Void
+macro am2()
 // expected-error@-1{{macro 'am2()' requires a definition}}
 
 #m1 + 1