Add regression test for fixed crasher

Author: slavapestov

validation-test/compiler_crashers_2_fixed/0175-sr8812.swift

@@ -0,0 +1,13 @@
+// RUN: not %target-swift-frontend -typecheck %s
+
+public protocol LocalizedError : Error {
+    var errorDescription: String? { get }
+}
+
+public enum AFError: Error {}
+extension AFError {
+    public var isInvalidURLError: Bool {
+
+extension AFError: LocalizedError {
+    public var errorDescription: String? {
+      return ""