[JSON] Unpoison memory before its reuse

This commit unpoisons memory before its reuse (with reinterpret_cast).
Required by #79049

Author: Advenam Tacet

llvm/include/llvm/Support/JSON.h

@@ -482,6 +482,12 @@ class Value {
   friend class Object;
 
   template <typename T, typename... U> void create(U &&... V) {
+#if defined(ADDRESS_SANITIZER) || defined(__SANITIZE_ADDRESS__)
+    // Unpoisoning to prevent overwriting poisoned object (e.g., annotated short string).
+    // Objects that have had their memory poisoned may cause an ASan error if their memory is reused
+    // without calling their destructor. Unpoisoning the memory prevents this error from occurring.
+    __asan_unpoison_memory_region(&Union, sizeof(T));
+#endif
     new (reinterpret_cast<T *>(&Union)) T(std::forward<U>(V)...);
   }
   template <typename T> T &as() const {