[analyzer] NonNullParamChecker: don't freak out about nested transparent_unions.

For now, just ignore them. Later, we could try looking through LazyCompoundVals,
but we at least shouldn't crash.

<rdar://problem/16153464>

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@202212 91177308-0d34-0410-b5e6-96231b3b80d8