Add more features.

Add `throws`/`rethrows` and trailing closures to feature list.
Features are tested in test/Sema/call_as_function_simple.swift.

Author: dan-zheng

CHANGELOG.md

@@ -46,6 +46,8 @@ Swift Next
   `func callAsFunction` argument labels are required at call sites.
   Multiple `func callAsFunction` methods on a single type are supported.
   `mutating func callAsFunction` is supported.
+  `func callAsFunction` works with `throws` and `rethrows`.
+  `func callAsFunction` works with trailing closures.
 
 * [SR-4206][]: