[ConstraintSystem] Move tracking of solver depth/states into SolverState.

Instead of doing this when scopes are created and destroyed, do this
when scopes are registered and rolled back on the SolverState.

NFC.

Author: rudkx

lib/Sema/CSSolver.cpp

@@ -2,7 +2,7 @@
 //
 // This source file is part of the Swift.org open source project
 //
-// Copyright (c) 2014 - 2017 Apple Inc. and the Swift project authors
+// Copyright (c) 2014 - 2018 Apple Inc. and the Swift project authors
 // Licensed under Apache License v2.0 with Runtime Library Exception
 //
 // See https://swift.org/LICENSE.txt for license information
@@ -472,8 +472,6 @@ ConstraintSystem::SolverState::~SolverState() {
 ConstraintSystem::SolverScope::SolverScope(ConstraintSystem &cs)
   : cs(cs), CGScope(cs.CG)
 {
-  ++cs.solverState->depth;
-
   resolvedOverloadSets = cs.resolvedOverloadSets;
   numTypeVariables = cs.TypeVariables.size();
   numSavedBindings = cs.solverState->savedBindings.size();
@@ -488,13 +486,9 @@ ConstraintSystem::SolverScope::SolverScope(ConstraintSystem &cs)
 
   cs.solverState->registerScope(this);
   assert(!cs.failedConstraint && "Unexpected failed constraint!");
-
-  ++cs.solverState->NumStatesExplored;
 }
 
 ConstraintSystem::SolverScope::~SolverScope() {
-  --cs.solverState->depth;
-
   // Erase the end of various lists.
   cs.resolvedOverloadSets = resolvedOverloadSets;
   truncate(cs.TypeVariables, numTypeVariables);

lib/Sema/ConstraintSystem.h

@@ -2,7 +2,7 @@
 //
 // This source file is part of the Swift.org open source project
 //
-// Copyright (c) 2014 - 2017 Apple Inc. and the Swift project authors
+// Copyright (c) 2014 - 2018 Apple Inc. and the Swift project authors
 // Licensed under Apache License v2.0 with Runtime Library Exception
 //
 // See https://swift.org/LICENSE.txt for license information
@@ -1150,6 +1150,9 @@ class ConstraintSystem {
     ///
     /// \param scope The scope to associate with current solver state.
     void registerScope(SolverScope *scope) {
+      ++depth;
+      ++NumStatesExplored;
+
       CS.incrementScopeCounter();
       auto scopeInfo =
         std::make_tuple(scope, retiredConstraints.begin(),
@@ -1212,6 +1215,8 @@ class ConstraintSystem {
     ///
     /// \param scope The solver scope to rollback.
     void rollback(SolverScope *scope) {
+      --depth;
+
       SolverScope *savedScope;
       // The position of last retired constraint before given scope.
       ConstraintList::iterator lastRetiredPos;