[CodeGen] Fix an assert in EmitNullConstant.
r235815 changed CGRecordLowering::accumulateBases to ignore non-virtual
bases of size 0, which prevented adding those non-virtual bases to
CGRecordLayout's NonVirtualBases. This caused clang to assert when
CGRecordLayout::getNonVirtualBaseLLVMFieldNo was called in
EmitNullConstant. This commit fixes the bug by ignoring zero-sized
non-virtual bases in EmitNullConstant.
rdar://problem/
28100139
Differential Revision: https://reviews.llvm.org/D24312
llvm-svn: 281405