[MC/DC] Refactor MCDC::State::Decision. NFC.

Introduce `ID` and `InvalidID`. Then `DecisionByStmt` can have three
states.

* Not assigned if the Stmt(Expr) doesn't exist.
* When `DecisionByStmt[Expr]` exists:
  * Invalid and should be ignored if `ID == Invalid`.
  * Valid if `ID != Invalid`. Other member will be filled in the
    Mapper.

Author: chapuni

clang/lib/CodeGen/CodeGenPGO.cpp

@@ -310,7 +310,7 @@ struct MapRegionCounters : public RecursiveASTVisitor<MapRegionCounters> {
           }
 
           // Otherwise, allocate the Decision.
-          MCDCState.DecisionByStmt[BinOp].BitmapIdx = 0;
+          MCDCState.DecisionByStmt[BinOp].ID = MCDCState.DecisionByStmt.size();
         }
         return true;
       }

clang/lib/CodeGen/CoverageMappingGen.cpp

@@ -2197,10 +2197,8 @@ struct CounterCoverageMappingBuilder
 
     // Update the state for CodeGenPGO
     assert(MCDCState.DecisionByStmt.contains(E));
-    MCDCState.DecisionByStmt[E] = {
-        MCDCState.BitmapBits, // Top
-        std::move(Builder.Indices),
-    };
+    MCDCState.DecisionByStmt[E].update(MCDCState.BitmapBits, // Top
+                                       std::move(Builder.Indices));
 
     auto DecisionParams = mcdc::DecisionParameters{
         MCDCState.BitmapBits += NumTVs, // Tail

clang/lib/CodeGen/MCDCState.h

@@ -16,6 +16,8 @@
 #include "llvm/ADT/DenseMap.h"
 #include "llvm/ADT/SmallVector.h"
 #include "llvm/ProfileData/Coverage/MCDCTypes.h"
+#include <cassert>
+#include <limits>
 
 namespace clang {
 class Stmt;
@@ -30,8 +32,20 @@ struct State {
   unsigned BitmapBits = 0;
 
   struct Decision {
+    using IndicesTy = llvm::SmallVector<std::array<int, 2>>;
+    static constexpr auto InvalidID = std::numeric_limits<unsigned>::max();
+
     unsigned BitmapIdx;
-    llvm::SmallVector<std::array<int, 2>> Indices;
+    IndicesTy Indices;
+    unsigned ID = InvalidID;
+
+    bool isValid() const { return ID != InvalidID; }
+
+    void update(unsigned I, IndicesTy &&X) {
+      assert(ID != InvalidID);
+      BitmapIdx = I;
+      Indices = std::move(X);
+    }
   };
 
   llvm::DenseMap<const Stmt *, Decision> DecisionByStmt;