Add reduced, crashing test case for rdar://problem/39130543.

Author: DougGregor

validation-test/compiler_crashers_2/0153-rdar39130543.swift

@@ -0,0 +1,21 @@
+// RUN: not --crash %target-swift-frontend %s -emit-ir
+
+protocol P20 { }
+
+protocol P21b {
+	associatedtype T
+
+	func foo(_: (T?) -> Void)
+}
+
+protocol P21a {
+	associatedtype T
+
+	func bar(_: ([T]) -> Void)
+}
+
+extension P21b where Self: P21a, T: P20 {
+	func foo(_: (T?) -> Void) {
+		bar { _ in }
+	}
+}