[ConstraintSystem] Add a way to get the elapsed time we've spent in a SolverScope.

Author: rudkx

lib/Sema/CSSolver.cpp

@@ -483,6 +483,9 @@ ConstraintSystem::SolverScope::SolverScope(ConstraintSystem &cs)
   numDefaultedConstraints = cs.DefaultedConstraints.size();
   numCheckedConformances = cs.CheckedConformances.size();
   PreviousScore = cs.CurrentScore;
+  if (cs.Timer) {
+    startTime = cs.Timer->getElapsedProcessTimeInFractionalSeconds();
+  }
 
   cs.solverState->registerScope(this);
   assert(!cs.failedConstraint && "Unexpected failed constraint!");

lib/Sema/ConstraintSystem.h

@@ -1445,6 +1445,9 @@ class ConstraintSystem {
     /// The previous score.
     Score PreviousScore;
 
+    /// Time in fractional seconds at which we entered this scope.
+    double startTime;
+
     /// Constraint graph scope associated with this solver scope.
     ConstraintGraphScope CGScope;
 
@@ -1456,6 +1459,13 @@ class ConstraintSystem {
   public:
     explicit SolverScope(ConstraintSystem &cs);
     ~SolverScope();
+
+    Optional<double> getElapsedTimeInFractionalSeconds() {
+      if (!cs.Timer)
+        return None;
+
+      return cs.Timer->getElapsedProcessTimeInFractionalSeconds() - startTime;
+    }
   };
 
   ConstraintSystem(TypeChecker &tc, DeclContext *dc,