[MC/DC] Refactor MCDCCoverageBuilder. NFC.
- Get rid of the old `DecisionStack` and dissolve it into push/pop
`CurCondIDs` in `VisitBin`, since `VisitBin` is recursive.
- Introduce the new `DecisionStack` with `DecisionState` to handle the
current `Decision` in nested `Decision`s.
- The stack has the sentinel that has `DecisionExpr = nullptr`.
- Split out `checkDecisionRootOrPush` from `pushAndAssignIDs` for
non-BinOp. It assigns `CondID` to `E` (instead of assignment LHS
in `pushAndAssignIDs`).
- The stack is manupilated at the top Decision operator in `VisitBin`.
- The stack grows at the entrance of the Decision with the initial
state.
- In the same level in `VisitBin`, the stack is popped and the
`Decision` record is emitted.
- Introduce `DecisionEndToSince` to sweep `MCDCBranch`es partially in
`cancelDecision`.
