[TypeChecker] Fix crash with Objective C keypaths.

We have a crash reported but do not have a test case and my attempt at
creating one was unsuccessful. It seems like the issue is that we have
an invalid decl that we attempt to access the interface type on,
though, so hopefully this fixes the issue.

I also tweaked the constraint generator to bail out when
checkObjCKeyPathExpr fails.

Potentially fixes: rdar://problem/40955755

(cherry picked from commit e0ce5fc)

Author: rudkx

lib/Sema/CSGen.cpp

@@ -2,7 +2,7 @@
 //
 // This source file is part of the Swift.org open source project
 //
-// Copyright (c) 2014 - 2017 Apple Inc. and the Swift project authors
+// Copyright (c) 2014 - 2018 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
@@ -3237,7 +3237,8 @@ namespace {
       if (auto keyPath = dyn_cast<KeyPathExpr>(expr)) {
         if (keyPath->isObjC()) {
           auto &cs = CG.getConstraintSystem();
-          (void)cs.getTypeChecker().checkObjCKeyPathExpr(cs.DC, keyPath);
+          if (!cs.getTypeChecker().checkObjCKeyPathExpr(cs.DC, keyPath))
+            return { false, nullptr };
         }
       }
 

lib/Sema/TypeCheckExprObjC.cpp

@@ -2,7 +2,7 @@
 //
 // This source file is part of the Swift.org open source project
 //
-// Copyright (c) 2014 - 2017 Apple Inc. and the Swift project authors
+// Copyright (c) 2014 - 2018 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
@@ -317,6 +317,8 @@ Optional<Type> TypeChecker::checkObjCKeyPathExpr(DeclContext *dc,
     // Handle property references.
     if (auto var = dyn_cast<VarDecl>(found)) {
       validateDecl(var);
+      if (var->isInvalid() || !var->hasInterfaceType())
+        return None;
 
       // Resolve this component to the variable we found.
       auto varRef = ConcreteDeclRef(var);