[ASan][ADT] Don't scribble with ASan (#79066)

With this commit, scribbling under AddressSanitizer (ASan) is disabled to prevent  overwriting poisoned objects (e.g., annotated short strings).
Needed by #79049

Author: Tacet

llvm/include/llvm/ADT/FunctionExtras.h

@@ -35,6 +35,7 @@
 #include "llvm/ADT/PointerIntPair.h"
 #include "llvm/ADT/PointerUnion.h"
 #include "llvm/ADT/STLForwardCompat.h"
+#include "llvm/Support/Compiler.h"
 #include "llvm/Support/MemAlloc.h"
 #include "llvm/Support/type_traits.h"
 #include <cstring>
@@ -317,8 +318,10 @@ template <typename ReturnT, typename... ParamTs> class UniqueFunctionBase {
     // Clear the old callback and inline flag to get back to as-if-null.
     RHS.CallbackAndInlineFlag = {};
 
-#ifndef NDEBUG
-    // In debug builds, we also scribble across the rest of the storage.
+#if !defined(NDEBUG) && !LLVM_ADDRESS_SANITIZER_BUILD
+    // In debug builds without ASan, we also scribble across the rest of the
+    // storage. Scribbling under AddressSanitizer (ASan) is disabled to prevent
+    // overwriting poisoned objects (e.g., annotated short strings).
     memset(RHS.getInlineStorage(), 0xAD, InlineStorageSize);
 #endif
   }