[silgenpattern] Fix some stack-use-after-free errors caused by iterating over an Optional<ArrayRef<T>>.

Specifically the bad pattern was:

```
   for (auto *vd : *caseStmt->getCaseBodyVariables()) { ... }
```

The problem is that the optional is not lifetime extended over the for loop. To
work around this, I changed the API of CaseStmt's getCaseBodyVariable methods to
never return the inner Optional<MutableArrayRef<T>>. Now we have the following 3
methods (ignoring const differences):

1. CaseStmt::hasCaseBodyVariables().

2. CaseStmt::getCaseBodyVariables(). Asserts if the case body variable array was
   never specified.

3. CaseStmt::getCaseBodyVariablesOrEmptyArray(). Returns either the case body
   variables array or an empty array if we were never given any case body
   variable array.

This should prevent anyone else in the future from hitting this type of bug.

radar://49609717

lib/SILGen/SILGenPattern.cpp

@@ -2332,12 +2332,8 @@ void PatternMatchEmission::initSharedCaseBlockDest(CaseStmt *caseBlock,
   auto *block = SGF.createBasicBlock();
   result.first->second.first = block;
 
-  // Add args for any pattern variables
-  auto caseBodyVars = caseBlock->getCaseBodyVariables();
-  if (!caseBodyVars)
-    return;
-
-  for (auto *vd : *caseBodyVars) {
+  // Add args for any pattern variables if we have any.
+  for (auto *vd : caseBlock->getCaseBodyVariablesOrEmptyArray()) {
     if (!vd->hasName())
       continue;
 
@@ -2365,14 +2361,10 @@ void PatternMatchEmission::emitAddressOnlyAllocations() {
   for (auto &entry : SharedCases) {
     CaseStmt *caseBlock = entry.first;
 
-    auto caseBodyVars = caseBlock->getCaseBodyVariables();
-    if (!caseBodyVars)
-      continue;
-
     // If we have a shared case with bound decls, setup the arguments for the
     // shared block by emitting the temporary allocation used for the arguments
     // of the shared block.
-    for (auto *vd : *caseBodyVars) {
+    for (auto *vd : caseBlock->getCaseBodyVariablesOrEmptyArray()) {
       if (!vd->hasName())
         continue;
 
@@ -2436,8 +2428,7 @@ void PatternMatchEmission::emitSharedCaseBlocks() {
     assert(SGF.getCleanupsDepth() == PatternMatchStmtDepth);
     SWIFT_DEFER { assert(SGF.getCleanupsDepth() == PatternMatchStmtDepth); };
 
-    auto caseBodyVars = caseBlock->getCaseBodyVariables();
-    if (!caseBodyVars) {
+    if (!caseBlock->hasCaseBodyVariables()) {
       emitCaseBody(caseBlock);
       continue;
     }
@@ -2448,7 +2439,7 @@ void PatternMatchEmission::emitSharedCaseBlocks() {
     // args needing Cleanup will get that as well.
     Scope scope(SGF.Cleanups, CleanupLocation(caseBlock));
     unsigned argIndex = 0;
-    for (auto *vd : *caseBodyVars) {
+    for (auto *vd : caseBlock->getCaseBodyVariables()) {
       if (!vd->hasName())
         continue;
 
@@ -2606,14 +2597,14 @@ static void switchCaseStmtSuccessCallback(SILGenFunction &SGF,
   if (!row.hasFallthroughTo() && caseBlock->getCaseLabelItems().size() == 1) {
     // If we have case body vars, set them up to point at the matching var
     // decls.
-    if (auto caseBodyVars = caseBlock->getCaseBodyVariables()) {
+    if (caseBlock->hasCaseBodyVariables()) {
       // Since we know that we only have one case label item, grab its pattern
       // vars and use that to update expected with the right SILValue.
       //
       // TODO: Do we need a copy here?
       SmallVector<VarDecl *, 4> patternVars;
       row.getCasePattern()->collectVariables(patternVars);
-      for (auto *expected : *caseBodyVars) {
+      for (auto *expected : caseBlock->getCaseBodyVariables()) {
         if (!expected->hasName())
           continue;
         for (auto *vd : patternVars) {
@@ -2640,8 +2631,7 @@ static void switchCaseStmtSuccessCallback(SILGenFunction &SGF,
 
   // If we do not have any bound decls, we do not need to setup any
   // variables. Just jump to the shared destination.
-  auto caseBodyVars = caseBlock->getCaseBodyVariables();
-  if (!caseBodyVars) {
+  if (!caseBlock->hasCaseBodyVariables()) {
     // Don't emit anything yet, we emit it at the cleanup level of the switch
     // statement.
     JumpDest sharedDest = emission.getSharedCaseBlockDest(caseBlock);
@@ -2658,7 +2648,7 @@ static void switchCaseStmtSuccessCallback(SILGenFunction &SGF,
   SILModule &M = SGF.F.getModule();
   SmallVector<VarDecl *, 4> patternVars;
   row.getCasePattern()->collectVariables(patternVars);
-  for (auto *expected : *caseBodyVars) {
+  for (auto *expected : caseBlock->getCaseBodyVariables()) {
     if (!expected->hasName())
       continue;
     for (auto *var : patternVars) {
@@ -2845,8 +2835,7 @@ void SILGenFunction::emitSwitchFallthrough(FallthroughStmt *S) {
 
   // If our destination case doesn't have any bound decls, there is no rebinding
   // to do. Just jump to the shared dest.
-  auto destCaseBodyVars = destCaseStmt->getCaseBodyVariables();
-  if (!destCaseBodyVars) {
+  if (!destCaseStmt->hasCaseBodyVariables()) {
     Cleanups.emitBranchAndCleanups(sharedDest, S);
     return;
   }
@@ -2856,13 +2845,13 @@ void SILGenFunction::emitSwitchFallthrough(FallthroughStmt *S) {
   SmallVector<SILValue, 4> args;
   CaseStmt *fallthroughSourceStmt = S->getFallthroughSource();
 
-  for (auto *expected : *destCaseBodyVars) {
+  for (auto *expected : destCaseStmt->getCaseBodyVariables()) {
     if (!expected->hasName())
       continue;
 
     // The type checker enforces that if our destination case has variables then
     // our fallthrough source must as well.
-    for (auto *var : *fallthroughSourceStmt->getCaseBodyVariables()) {
+    for (auto *var : fallthroughSourceStmt->getCaseBodyVariables()) {
       if (!var->hasName() || var->getName() != expected->getName()) {
         continue;
       }