[llvm] [clang] [clang][CoverageMapping] Refactor when setting MC/DC True/False (PR #78202)

[Alan Phipps via cfe-commits]

================
@@ -676,41 +679,25 @@ struct MCDCCoverageBuilder {
     return E->getOpcode() == BO_LAnd;
   }
 
-  /// Push an ID onto the corresponding RHS stack.
-  void pushRHS(const BinaryOperator *E) {
-    llvm::SmallVector<MCDCConditionID> &rhs = isLAnd(E) ? AndRHS : OrRHS;
-    rhs.push_back(CondIDs[CodeGenFunction::stripCond(E->getRHS())]);
-  }
-
-  /// Pop an ID from the corresponding RHS stack.
-  void popRHS(const BinaryOperator *E) {
-    llvm::SmallVector<MCDCConditionID> &rhs = isLAnd(E) ? AndRHS : OrRHS;
-    if (!rhs.empty())
-      rhs.pop_back();
-  }
-
-  /// If the expected ID is on top, pop it off the corresponding RHS stack.
-  void popRHSifTop(const BinaryOperator *E) {
-    if (!OrRHS.empty() && CondIDs[E] == OrRHS.back())
-      OrRHS.pop_back();
-    else if (!AndRHS.empty() && CondIDs[E] == AndRHS.back())
-      AndRHS.pop_back();
-  }
-
 public:
   MCDCCoverageBuilder(CodeGenModule &CGM,
                       llvm::DenseMap<const Stmt *, MCDCConditionID> &CondIDMap,
                       llvm::DenseMap<const Stmt *, unsigned> &MCDCBitmapMap)
-      : CGM(CGM), CondIDs(CondIDMap), MCDCBitmapMap(MCDCBitmapMap) {}
+      : CGM(CGM), DecisionStack(1), CondIDs(CondIDMap),
----------------
evodius96 wrote:

Rather than define a `DecisionStackSentinel` equal to 1, I'd prefer to define a `constexpr` sentinel value that is explicitly initialized to [0,0] (even though this is the default value).  Then it use that to explicitly initialize the stack.
```
static constexpr DecisionIDPair DecisionIDPairSentinel{0,0};
...
DecisionStack(1, DecisionIDPairSentinel);
```
I think this makes it clear what is being done and keeps DecisionIDPair relatively straightforward.

https://github.com/llvm/llvm-project/pull/78202