[analyzer] Add tests for symbolic expression liveness.

There are very few tests here because SValBuilder is fairly aggressive
about not building SymExprs that we can't evaluate, which saves memory
and CPU but also makes it very much tied to the current constraint
manager. We should probably scale back here and let things decay to
UnknownVal later on.

bitwise-ops.c tests that for the SymExprs we do create, we persist our
assumptions about them. traversal-path-unification.c tests that we do
clean out constraints on arbitrary SymExprs once they have actually died.

llvm-svn: 164623