Analysis: Add a CFG successor to a SwitchStmt if it is both empty and fully covered

Consider the case where a SwitchStmt satisfied isAllEnumCasesCovered()
as well as having no cases at all (i.e. the enum it covers has no
enumerators).

In this case, we should add a successor to repair the CFG.

This fixes PR16212.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@183237 91177308-0d34-0410-b5e6-96231b3b80d8