Add Optional callAsFunction test.

Author: dan-zheng

test/Sema/call_as_function_optional.swift

@@ -0,0 +1,19 @@
+// RUN: %target-typecheck-verify-swift
+
+// Test extending `Optional` with `func callAsFunction`.
+// `Optional` is an edge case since constraint simplification has special
+// `Optional` stripping logic.
+
+extension Optional {
+  func callAsFunction() -> Optional {
+    return self
+  }
+  func callAsFunction(_ fn: (Int) -> Void) -> Optional {
+    return self
+  }
+}
+func testOptional<T>(_ x: T?) {
+  _ = x()()()
+  // Test trailing closure syntax.
+  _ = x { _ in } ({ _ in })
+}