TBAA: make sure zero-length bitfield works for tbaa.struct and path-aware tbaa

For ms structs, zero-length bitfields following non-bitfield members are
completely ignored, we should not increase the field index.
Before the fix, we will have an assertion failure.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@180038 91177308-0d34-0410-b5e6-96231b3b80d8