Revert "CodeGen: Simplify CodeGenFunction::EmitCaseStmt"

I misunderstood the discussion on this. The complexity here is
justified by the malloc overhead it saves.

This reverts commit r199302.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@199700 91177308-0d34-0410-b5e6-96231b3b80d8