Make switchBuilder initialization clearer (#5957)

Co-authored-by: Werner Dietl <wdietl@gmail.com>

Author: mernst

dataflow/src/main/java/org/checkerframework/dataflow/cfg/builder/CFGTranslationPhaseOne.java

@@ -580,8 +580,11 @@ public Node visitYield17(Tree yieldTree, Void p) {
  * @return the result of visiting the switch expression tree
  */
  public Node visitSwitchExpression17(Tree switchExpressionTree, Void p) {
- SwitchBuilder switchBuilder = new SwitchBuilder(switchExpressionTree);
- return switchBuilder.build();
+ SwitchBuilder oldSwitchBuilder = switchBuilder;
+ switchBuilder = new SwitchBuilder(switchExpressionTree);
+ Node result = switchBuilder.build();
+ switchBuilder = oldSwitchBuilder;
+ return result;
  }
 
  /**
@@ -2319,8 +2322,6 @@ private SwitchBuilder(Tree switchTree) {
  * null
  */
  public @Nullable SwitchExpressionNode build() {
- SwitchBuilder oldSwitchBuilder = switchBuilder;
- switchBuilder = this;
  LabelCell oldBreakTargetLC = breakTargetLC;
  breakTargetLC = new LabelCell(new Label());
  int numCases = caseTrees.size();
@@ -2382,7 +2383,6 @@ private SwitchBuilder(Tree switchTree) {
  env.getTypeUtils()));
  }
 
- switchBuilder = oldSwitchBuilder;
  if (!TreeUtils.isSwitchStatement(switchTree)) {
  // It's a switch expression, not a switch statement.
  IdentifierTree switchExprVarUseTree = treeBuilder.buildVariableUse(switchExprVarTree);