Cope with ASTScope being disabled in statement checker.

Code completion still disables ASTScope lookup, so fall back to using
the old state in such cases. We'll remove all of this code once code
completion (and LLDB?) move off of the old lookup mechanisms.

Author: DougGregor

lib/Sema/TypeCheckStmt.cpp

@@ -302,6 +302,11 @@ static bool isDefer(Optional<AnyFunctionRef> enclosingFunc) {
 /// same label.
 static void checkLabeledStmtShadowing(
     ASTContext &ctx, SourceFile *sourceFile, LabeledStmt *ls) {
+  // If ASTScope lookup is disabled, don't do this check at all.
+  // FIXME: Enable ASTScope lookup everywhere.
+  if (!ctx.LangOpts.EnableASTScopeLookup)
+    return;
+
   auto name = ls->getLabelInfo().Name;
   if (name.empty() || !sourceFile || ls->getStartLoc().isInvalid())
     return;
@@ -355,11 +360,23 @@ static LabeledStmt *findBreakOrContinueStmtTarget(
     ASTContext &ctx, SourceFile *sourceFile,
     SourceLoc loc, Identifier targetName, SourceLoc targetLoc,
     bool isContinue,
-    Optional<AnyFunctionRef> enclosingFunc) {
+    Optional<AnyFunctionRef> enclosingFunc,
+    ArrayRef<LabeledStmt *> oldActiveLabeledStmts) {
   TopCollection<unsigned, LabeledStmt *> labelCorrections(3);
 
+  // Retrieve the active set of labeled statements.
+  // FIXME: Once everything uses ASTScope lookup, \c oldActiveLabeledStmts
+  // can go away.
+  SmallVector<LabeledStmt *, 4> activeLabeledStmts;
+  if (ctx.LangOpts.EnableASTScopeLookup) {
+    activeLabeledStmts = ASTScope::lookupLabeledStmts(sourceFile, loc);
+  } else {
+    activeLabeledStmts.insert(
+        activeLabeledStmts.end(),
+        oldActiveLabeledStmts.rbegin(), oldActiveLabeledStmts.rend());
+  }
+
   // Pick the nearest break target that matches the specified name.
-  auto activeLabeledStmts = ASTScope::lookupLabeledStmts(sourceFile, loc);
   if (targetName.empty()) {
     for (auto labeledStmt : activeLabeledStmts) {
       // 'break' with no label looks through non-loop structures
@@ -481,7 +498,9 @@ class StmtChecker : public StmtVisitor<StmtChecker, Stmt*> {
 
       // Verify that the ASTScope-based query for active labeled statements
       // is equivalent to what we have here.
-      if (LS->getStartLoc().isValid() && sourceFile) {
+      if (LS->getStartLoc().isValid() && sourceFile &&
+          SC.getASTContext().LangOpts.EnableASTScopeLookup &&
+          !SC.getASTContext().Diags.hadAnyError()) {
         // The labeled statements from ASTScope lookup have the
         // innermost labeled statement first, so reverse it to
         // match the data structure maintained here.
@@ -858,7 +877,7 @@ class StmtChecker : public StmtVisitor<StmtChecker, Stmt*> {
     if (auto target = findBreakOrContinueStmtTarget(
             getASTContext(), DC->getParentSourceFile(), S->getLoc(),
             S->getTargetName(), S->getTargetLoc(), /*isContinue=*/false,
-            TheFunc)) {
+            TheFunc, ActiveLabeledStmts)) {
       S->setTarget(target);
     }
 
@@ -869,7 +888,7 @@ class StmtChecker : public StmtVisitor<StmtChecker, Stmt*> {
     if (auto target = findBreakOrContinueStmtTarget(
             getASTContext(), DC->getParentSourceFile(), S->getLoc(),
             S->getTargetName(), S->getTargetLoc(), /*isContinue=*/true,
-            TheFunc)) {
+            TheFunc, ActiveLabeledStmts)) {
       S->setTarget(target);
     }
 
@@ -878,15 +897,21 @@ class StmtChecker : public StmtVisitor<StmtChecker, Stmt*> {
 
   Stmt *visitFallthroughStmt(FallthroughStmt *S) {
     auto sourceFile = DC->getParentSourceFile();
-    auto activeLabeledStmts = ASTScope::lookupLabeledStmts(
-        sourceFile, S->getLoc());
-    auto numSwitches = llvm::count_if(activeLabeledStmts,
-                                      [](LabeledStmt *labeledStmt) {
-      return isa<SwitchStmt>(labeledStmt);
-    });
-    assert(numSwitches == SwitchLevel);
+    bool hasAnySwitches;
+    if (getASTContext().LangOpts.EnableASTScopeLookup) {
+      auto activeLabeledStmts = ASTScope::lookupLabeledStmts(
+          sourceFile, S->getLoc());
+      auto numSwitches = llvm::count_if(activeLabeledStmts,
+                                        [](LabeledStmt *labeledStmt) {
+        return isa<SwitchStmt>(labeledStmt);
+      });
+      assert(numSwitches == SwitchLevel);
+      hasAnySwitches = numSwitches > 0;
+    } else {
+      hasAnySwitches = SwitchLevel > 0;
+    }
 
-    if (!numSwitches) {
+    if (!hasAnySwitches) {
       getASTContext().Diags.diagnose(S->getLoc(),
                                      diag::fallthrough_outside_switch);
       return nullptr;