[ConstraintSystem] Test @autoclosure behavior in Swift < 5 compatibility mode

Swift versions < 5 allowed argument passed to @autoclosure parameter
to be @autoclosure function type, we need to make sure that such
behavior is preserved.

Author: xedin

test/Compatibility/attr_autoclosure.swift

@@ -0,0 +1,39 @@
+// RUN: %target-swift-frontend -emit-sil -verify %s | %FileCheck %s
+// REQUIRES: SWIFT_VERSION=4
+
+do {
+  func a(_ x: @autoclosure () -> Int) {}
+  func b(_ x: Int, _ f: @autoclosure () -> Int = #line) {}
+
+  func c(_ y: @autoclosure () -> Int, _ z: Int) {
+    // CHECK: function_ref @$s16attr_autoclosure1aL_yySiyXKF
+    a(y)
+
+    // CHECK: function_ref @$s16attr_autoclosure1cL_yySiyXK_SitFSiyXEfu_
+    b(z + 42) // ok
+    b(z, y) // ok to pass `@autoclosure` function type as an argument
+    b(z, y()) // ok
+  }
+}
+
+func foo(_ f: @autoclosure () -> Int) {}
+func foo(_ f: () -> Int) {}
+
+do {
+  func bar(_ a: @autoclosure () -> Int,
+           _ b: () -> Int,
+           _ c: Int) {
+    // CHECK: function_ref @$s16attr_autoclosure3fooyySiyXEF
+    foo(a)
+    // CHECK: function_ref @$s16attr_autoclosure3fooyySiyXEF
+    foo(b)
+
+    // CHECK: function_ref @$s16attr_autoclosure3fooyySiyXKF
+    foo(a())
+    // CHECK: function_ref @$s16attr_autoclosure3fooyySiyXKF
+    foo(b())
+
+    // CHECK: function_ref @$s16attr_autoclosure3fooyySiyXKF
+    foo(c)
+  }
+}