[CodeGen] Fix an assert in EmitNullConstant

[ahatanak]

Summary

r235815 changed CGRecordLowering::accumulateBases to ignore non-virtual bases of size 0, which prevented adding those non-virtual bases to CGRecordLayout's NonVirtualBases. EmitNullConstant calls CGRecordLayout::getNonVirtualBaseLLVMFieldNo to get the field number of the base, which causes an assert. This patch fixes the bug.

[rjmccall]

LGTM.

lib/CodeGen/CGExprConstant.cpp

if (I.isVirtual()) {
       // object, we'll lay these out later.
       continue;
     }
 
     const CXXRecordDecl *base = 
       cast<CXXRecordDecl>(I.getType()->castAs<RecordType>()->getDecl());
 
     // Ignore empty bases.
-    if (base->isEmpty())
+    if (base->isEmpty() ||
+        CGM.getContext().getASTRecordLayout(base).getNonVirtualSize().isZero())
       continue;
     
     unsigned fieldIndex = layout.getNonVirtualBaseLLVMFieldNo(base);
     llvm::Type *baseType = structure->getElementType(fieldIndex);
     elements[fieldIndex] = EmitNullConstantForBase(CGM, baseType, base);
   }
 
   // Fill in all the fields.

test/CodeGenCXX/empty-classes.cpp

namespace rdar20621065 {
   struct B : A {
     float left;
     float right;
   };
 
   // Type checked at the top of the file.
   B b;
 };
+
+// This test used to crash when CGRecordLayout::getNonVirtualBaseLLVMFieldNo was called.
+namespace record_layout {
+struct X0 {
+  int x[0];
+};
+
+template<typename>
+struct X2 : X0 {
+};
+
+template<typename>
+struct X3 : X2<int> {
+  X3() : X2<int>() {}
+};
+
+
+void test0() {
+  X3<int>();
+}
+}