Add the ability for the Context to "forget" about slab allocations.

This is useful for projects that create one long-lived Context object for all
their GTIRB needs, and the object is destroyed at program shutdown. That
scenario results in doing a lot of wasted effort deallocating objects when the
OS will be reclaiming that memory anyway.

include/gtirb/Allocator.hpp

@@ -301,6 +301,15 @@ template <typename T> class SpecificBumpPtrAllocator {
   /// Allocate space for an array of objects without constructing them.
   T* Allocate(size_t num = 1) { return Allocator.Allocate<T>(num); }
 
+  /// Forgets all allocations from the underlying allocator, effectively
+  /// leaking the memory. This is useful when the allocator is no longer needed
+  /// and the operating system will be reclaiming the memory (such as at
+  // program shutdown time).
+  void ForgetAllocations() {
+    Allocator.Slabs.clear();
+    Allocator.CustomSizedSlabs.clear();
+  }
+
 private:
   /// Call the destructor of each allocated object and deallocate all but the
   /// current slab and reset the current pointer to the beginning of it, freeing

include/gtirb/Context.hpp

@@ -110,6 +110,11 @@ class GTIRB_EXPORT_API Context {
   Context();
   ~Context();
 
+  /// \brief Forgets all arena allocations held by this \ref Context object.
+  /// This can be useful under circumstances where leaking the memory is
+  /// acceptable, such as when shutting a program down.
+  void ForgetAllocations();
+
   /// \brief Create an object of type \ref T.
   ///
   /// \tparam NodeTy   The type of object for which to allocate memory.

src/Context.cpp

@@ -33,6 +33,18 @@ Context::~Context() = default;
 
 void Context::unregisterNode(const Node* N) { UuidMap.erase(N->getUUID()); }
 
+void Context::ForgetAllocations() {
+  NodeAllocator.ForgetAllocations();
+  CodeBlockAllocator.ForgetAllocations();
+  ByteIntervalAllocator.ForgetAllocations();
+  DataBlockAllocator.ForgetAllocations();
+  IrAllocator.ForgetAllocations();
+  ModuleAllocator.ForgetAllocations();
+  ProxyBlockAllocator.ForgetAllocations();
+  SectionAllocator.ForgetAllocations();
+  SymbolAllocator.ForgetAllocations();
+}
+
 const Node* Context::findNode(const UUID& ID) const {
   auto Iter = UuidMap.find(ID);
   return Iter != UuidMap.end() ? Iter->second : nullptr;