[analyzer] Generate a LazyCompoundVal when loading from a union-typed region.

This ensures that variables accessible through a union are invalidated when
the union value is passed to a function. We still don't fully handle union
values, but this should at least quiet some false positives.

PR16596

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@193265 91177308-0d34-0410-b5e6-96231b3b80d8