Fix LetPropertiesOpt crash on @_objcImplementation extensions

Fixes rdar://115139065 (Swift Compiler; @_objcImplementation; crash in
LetPropertiesOpt)

Author: atrick

lib/SILOptimizer/IPO/LetPropertiesOpts.cpp

@@ -239,7 +239,8 @@ void LetPropertiesOpt::optimizeLetPropertyAccess(VarDecl *Property,
     return;
 
   auto *Ty = dyn_cast<NominalTypeDecl>(Property->getDeclContext());
-  if (SkipTypeProcessing.count(Ty))
+  // Ty is null for properties declared inside an extension of an ObjC type.
+  if (!Ty || SkipTypeProcessing.count(Ty))
     return;
 
   LLVM_DEBUG(llvm::dbgs() << "Replacing access to property '" << *Property

test/SILOptimizer/Inputs/let_properties_opts.h

@@ -3,3 +3,5 @@
 @interface ObjcInterface: NSObject
 @end
 
+@interface ObjcInterfaceConstInit: NSObject
+@end

test/SILOptimizer/let_properties_opts_objc.swift

@@ -11,8 +11,19 @@
 
 // CHECK-LABEL: sil @$s4test0A13ObjcInterfaceySiSo0bC0CF
 // CHECK:         ref_element_addr [immutable] %0 : $ObjcInterface, #ObjcInterface.i
-// CHECK:       } // end sil function '$s4test0A13ObjcInterfaceySiSo0bC0CF'
+// CHECK-LABEL: } // end sil function '$s4test0A13ObjcInterfaceySiSo0bC0CF'
 public func testObjcInterface(_ x: ObjcInterface) -> Int {
   return x.i
 }
 
+// Test optimization of a private constant. This constant must be declared in a separate type without other fields.
+@_objcImplementation extension ObjcInterfaceConstInit {
+  private let constant: Int = 0
+
+  // CHECK-LABEL: sil hidden @$sSo22ObjcInterfaceConstInitC4testE0E15PrivateConstantSiyF : $@convention(method) (@guaranteed ObjcInterfaceConstInit) -> Int {
+  // CHECK: ref_element_addr [immutable] %0 : $ObjcInterfaceConstInit, #ObjcInterfaceConstInit.constant
+  // CHECK-LABEL: } // end sil function '$sSo22ObjcInterfaceConstInitC4testE0E15PrivateConstantSiyF'
+  final func testPrivateConstant() -> Int {
+    return constant
+  }
+}