Previously each of these `switch` statements would implicitly do
nothing if an unanticipated `enum` value came along. My impression is
that each of these `switch` statements is supposed to be exhaustive,
such that an unexpected (unhandled) value should never appear. If one
does, we should recognize it and complain loudly.
Of course, sometimes the right behavior for previously-unhandled
values is to do nothing. It may not always be clear whether an
exception or doing nothing is the right choice. For this commit,
WALA's regression tests still pass even with the possibility of
throwing an exception for unexpected values. If we assume that the
test suite is thorough, that tells me that throwing an exception is
the right policy for each `switch` statement that I'm changing here.
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