Sync with https://github.com/practicalswift/swift-compiler-crashes

Add 5 compiler crashes (+ 1 fixed crash).

validation-test/compiler_crashers/24879-getmemberforbasetype.swift

@@ -0,0 +1,11 @@
+// RUN: not --crash %target-swift-frontend %s -parse
+
+// Distributed under the terms of the MIT license
+// Test case submitted to project by https://github.com/zneak (zneak)
+
+protocol A { typealias B }
+class C : A { typealias B = Int }
+
+func crash<D: C>() -> Bool {
+	let a: D.B? = nil
+}

validation-test/compiler_crashers/26813-generic-enum-tuple-optional-payload.swift

@@ -0,0 +1,13 @@
+// RUN: not --crash %target-swift-frontend %s -parse
+
+// Distributed under the terms of the MIT license
+// Test case submitted to project by https://github.com/austinzheng (Austin Zheng)
+
+enum A<T> {
+    case Just(T)
+    case Error
+}
+
+func foo() -> A<(String, String?)> {
+    return A.Just("abc", "def")
+}

validation-test/compiler_crashers/28184-swift-constraints-constraintsystem-gettypeofmemberreference.swift

@@ -0,0 +1,14 @@
+// RUN: not --crash %target-swift-frontend %s -parse
+// REQUIRES: objc_interop
+
+// Distributed under the terms of the MIT license
+// Test case found by https://github.com/benshan (Ben Shanfelder)
+
+import Foundation
+
+extension NSObject {
+    var handler: Int {
+        // FIXME: Crashing due to lack of get {}
+        set {}
+    }
+}

validation-test/compiler_crashers_fixed/26296-duplicate-subscript-declaration.swift

@@ -0,0 +1,15 @@
+// RUN: not %target-swift-frontend %s -parse
+
+// rdar://22007370
+
+class Foo {
+    subscript(key: String) -> String {
+        get { a }
+        set { b }
+    }
+
+    subscript(key: String) -> String {
+        get { a }
+        set { b }
+    }
+}