[clang][bytecode] Create InterpState allocator on demand (#158802)

We often don't need it (especially in C), so make this optional and
create it only when we first allocate something.

Author: tbaederr

clang/lib/AST/ByteCode/InterpState.h

@@ -122,7 +122,9 @@ class InterpState final : public State, public SourceMapper {
   StdAllocatorCaller getStdAllocatorCaller(StringRef Name) const;
 
   void *allocate(size_t Size, unsigned Align = 8) const {
-    return Allocator.Allocate(Size, Align);
+    if (!Allocator)
+      Allocator.emplace();
+    return Allocator->Allocate(Size, Align);
   }
   template <typename T> T *allocate(size_t Num = 1) const {
     return static_cast<T *>(allocate(Num * sizeof(T), alignof(T)));
@@ -188,7 +190,7 @@ class InterpState final : public State, public SourceMapper {
   /// for.
   llvm::SmallVector<const Block *> InitializingBlocks;
 
-  mutable llvm::BumpPtrAllocator Allocator;
+  mutable std::optional<llvm::BumpPtrAllocator> Allocator;
 };
 
 class InterpStateCCOverride final {