Add a few more type-checker crasher addressed by the "occurs" check.

For rdar://problem/19343997, rdar://problem/19924870, and rdar://problem/20771765.

Author: DougGregor

validation-test/Sema/type_checker_crashers_fixed/rdar19343997.swift

@@ -0,0 +1,16 @@
+// RUN: not %target-swift-frontend %s -parse
+
+func ignore<T>(_ parser: (String) -> (T, String)?) -> (String) -> ((), String)? {
+	return { parser($0).map { _ in () } }
+}
+func | <T> (left: (String) -> (T, String)?, right: (String) -> ((), String)?) -> (String) -> (T?, String)? {
+	return { _ in nil }
+}
+
+ignore(" " | "\r" | "\t" | "\n")
+
+// Related: rdar://problem/19924870
+func unit<T>(_ x: T) -> T? { return x }
+func f() -> Int? {
+	return unit(1) ?? unit(2).map { 1 } ?? nil
+}

validation-test/Sema/type_checker_crashers_fixed/rdar20771765.swift

@@ -0,0 +1,22 @@
+// RUN: %target-swift-frontend %s -parse
+
+enum Pattern {
+case Specific(title: String?, label: String?, value: AnyObject?)
+case General(String)
+}
+
+extension Pattern : CustomDebugStringConvertible {
+  var debugDescription: String {
+    switch(self) {
+    case let .Specific(title, label, value):
+      let elements = [
+        title.map { "title: \(String(reflecting: $0))" },
+        label.map { "label: \(String(reflecting: $0))" },
+        value.map { "value: \(String(reflecting: $0))" }
+      ].flatMap { $0 }
+      return "x"
+    case let .General(s):
+      return ".General(\(String(reflecting: s))))"
+    }
+  }
+}