[clang][bytecode] Lazily create DynamicAllocator (#155831)

Due to all the tracking via map(s) and a BumpPtrAllocator, the creating
and destroying the DynamicAllocator is rather expensive. Try to do it
lazily and only create it when first calling
InterpState::getAllocator().

clang/lib/AST/ByteCode/InterpState.cpp

@@ -59,7 +59,8 @@ InterpState::~InterpState() {
 void InterpState::cleanup() {
   // As a last resort, make sure all pointers still pointing to a dead block
   // don't point to it anymore.
-  Alloc.cleanup();
+  if (Alloc)
+    Alloc->cleanup();
 }
 
 Frame *InterpState::getCurrentFrame() { return Current; }
@@ -99,10 +100,13 @@ void InterpState::deallocate(Block *B) {
 }
 
 bool InterpState::maybeDiagnoseDanglingAllocations() {
-  bool NoAllocationsLeft = !Alloc.hasAllocations();
+  if (!Alloc)
+    return true;
+
+  bool NoAllocationsLeft = !Alloc->hasAllocations();
 
   if (!checkingPotentialConstantExpression()) {
-    for (const auto &[Source, Site] : Alloc.allocation_sites()) {
+    for (const auto &[Source, Site] : Alloc->allocation_sites()) {
       assert(!Site.empty());
 
       CCEDiag(Source->getExprLoc(), diag::note_constexpr_memory_leak)

clang/lib/AST/ByteCode/InterpState.h

@@ -115,7 +115,13 @@ public:
 
   void setEvalLocation(SourceLocation SL) { this->EvalLocation = SL; }
 
-  DynamicAllocator &getAllocator() { return Alloc; }
+  DynamicAllocator &getAllocator() {
+    if (!Alloc) {
+      Alloc = std::make_unique<DynamicAllocator>();
+    }
+
+    return *Alloc.get();
+  }
 
   /// Diagnose any dynamic allocations that haven't been freed yet.
   /// Will return \c false if there were any allocations to diagnose,
@@ -161,7 +167,7 @@ private:
   /// Reference to the offset-source mapping.
   SourceMapper *M;
   /// Allocator used for dynamic allocations performed via the program.
-  DynamicAllocator Alloc;
+  std::unique_ptr<DynamicAllocator> Alloc;
 
 public:
   /// Reference to the module containing all bytecode.