Merge pull request #3138 from practicalswift/swiftc-28326-swift-typebase-getmembersubstitutions

[swiftc] Add 💥 case (😢 → 47, 😀 → 5089) triggered in swift::TypeBase::getMemberSubstitutions(…)

Author: gribozavr

validation-test/compiler_crashers/28326-swift-typebase-getmembersubstitutions.swift

@@ -0,0 +1,9 @@
+// This source file is part of the Swift.org open source project
+// Copyright (c) 2014 - 2016 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See http://swift.org/LICENSE.txt for license information
+// See http://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+
+// RUN: not --crash %target-swift-frontend %s -parse
+class d{init(){{extension{{}protocol a{struct B{let a