[GSB] Don't crash when substitution fails to produce a type.

Fixes two more recently-found GSB crashers.

Author: DougGregor

lib/AST/GenericSignatureBuilder.cpp

@@ -3704,7 +3704,7 @@ ConstraintResult GenericSignatureBuilder::addRequirement(
                                              ModuleDecl *inferForModule) {
   auto subst = [&](Type t) {
     if (subMap)
-      return t.subst(*subMap);
+      return t.subst(*subMap, SubstFlags::UseErrorType);
 
     return t;
   };

validation-test/compiler_crashers_fixed/28802-constrainttype-missing-constraint-type.swift

@@ -0,0 +1,10 @@
+// This source file is part of the Swift.org open source project
+// Copyright (c) 2014 - 2017 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+
+// REQUIRES: asserts
+// RUN: not %target-swift-frontend %s -emit-ir
+class a<a{protocol A{protocol A{}typealias e where f:A

validation-test/compiler_crashers_fixed/28804-second.swift

@@ -0,0 +1,12 @@
+// This source file is part of the Swift.org open source project
+// Copyright (c) 2014 - 2017 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+
+// REQUIRES: asserts
+// RUN: not %target-swift-frontend %s -emit-ir
+class a<a{
+protocol A{{}typealias e where f=A
+protocol A