Ban Placeholders in More Places

Fix a regression introduced in #39627 where placeholders at the "top level" were mistaken for "placeholders in top level code".

test/Constraints/diagnostics.swift

@@ -1138,6 +1138,7 @@ func badTypes() {
 func unresolvedTypeExistential() -> Bool {
   return (Int.self==_{})
   // expected-error@-1 {{type of expression is ambiguous without more context}}
+  // expected-error@-2 {{type placeholder not allowed here}}
 }
 
 do {