[analyzer] Add BranchCondition callback to 'switch' (#182058)
Previously the condition of a 'switch' statement did not trigger a
`BranchCondition` callback. This commit resolves this old FIXME and e.g.
lets the checker `core.uninitialzed.Branch` report code where the
condition of a `switch` statement is undefined.
This commit also contains a very small unrelated change that removes a
short fragment of dead code from `processBranch`.