Prevent copying of ArenaAllocator (#97935)

Summary:
This patch removes copy constructor and assignment operator from
ArenaAllocator class to prevent resource duplication and ensure it
remains non-copyable as per design intent.

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Differential Revision: https://phabricator.intern.facebook.com/D60251510

Author: smanna12

llvm/include/llvm/Demangle/MicrosoftDemangle.h

@@ -54,6 +54,10 @@ class ArenaAllocator {
     }
   }
 
+  // Delete the copy constructor and the copy assignment operator.
+  ArenaAllocator(const ArenaAllocator &) = delete;
+  ArenaAllocator &operator=(const ArenaAllocator &) = delete;
+
   char *allocUnalignedBuffer(size_t Size) {
     assert(Head && Head->Buf);