[Constraint system] Sink down debug logging.

This way we’ll see constraints whenever they’re generated, and remove some
of the expression-centric nature of the top-level solve() and solveImpl().

Author: DougGregor

lib/Sema/CSApply.cpp

@@ -7441,6 +7441,13 @@ Optional<SolutionApplicationTarget> ConstraintSystem::applySolution(
 
     solution.setExprTypes(resultExpr);
     result.setExpr(resultExpr);
+
+    if (Context.TypeCheckerOpts.DebugConstraintSolver) {
+      auto &log = Context.TypeCheckerDebug->getStream();
+      log << "---Type-checked expression---\n";
+      resultExpr->dump(log);
+      log << "\n";
+    }
   }
 
   rewriter.finalize();

lib/Sema/CSGen.cpp

@@ -3947,6 +3947,14 @@ bool ConstraintSystem::generateConstraints(
       return true;
     }
 
+    if (getASTContext().TypeCheckerOpts.DebugConstraintSolver) {
+      auto &log = getASTContext().TypeCheckerDebug->getStream();
+      log << "---Initial constraints for the given expression---\n";
+      print(log, expr);
+      log << "\n";
+      print(log);
+    }
+
     return false;
   }
 

lib/Sema/CSSolver.cpp

@@ -1252,17 +1252,10 @@ ConstraintSystem::solveImpl(SolutionApplicationTarget &target,
     return SolutionResult::forError();;
 
   // Notify the listener that we've built the constraint system.
-  Expr *expr = target.getAsExpr();
-  if (listener && listener->builtConstraints(*this, expr)) {
-    return SolutionResult::forError();
-  }
-
-  if (getASTContext().TypeCheckerOpts.DebugConstraintSolver) {
-    auto &log = getASTContext().TypeCheckerDebug->getStream();
-    log << "---Initial constraints for the given expression---\n";
-    print(log, expr);
-    log << "\n";
-    print(log);
+  if (Expr *expr = target.getAsExpr()) {
+    if (listener && listener->builtConstraints(*this, expr)) {
+      return SolutionResult::forError();
+    }
   }
 
   // Try to solve the constraint system using computed suggestions.
@@ -1272,8 +1265,6 @@ ConstraintSystem::solveImpl(SolutionApplicationTarget &target,
   if (getExpressionTooComplex(solutions))
     return SolutionResult::forTooComplex();
 
-  target.setExpr(expr);
-
   switch (solutions.size()) {
   case 0:
     return SolutionResult::forUndiagnosedError();

lib/Sema/TypeCheckConstraints.cpp

@@ -2132,13 +2132,6 @@ TypeChecker::typeCheckExpression(
       return None;
   }
 
-  if (Context.TypeCheckerOpts.DebugConstraintSolver) {
-    auto &log = Context.TypeCheckerDebug->getStream();
-    log << "---Type-checked expression---\n";
-    result->dump(log);
-    log << "\n";
-  }
-
   // Unless the client has disabled them, perform syntactic checks on the
   // expression now.
   if (!cs.shouldSuppressDiagnostics() &&