[libc] Only perform MSAN unpoison in non-constexpr context. (#72299)

Author: lntue

libc/src/__support/macros/sanitizer.h

@@ -47,9 +47,16 @@
 // Functions to unpoison memory
 //-----------------------------------------------------------------------------
 
-#ifdef LIBC_HAVE_MEMORY_SANITIZER
+#if defined(LIBC_HAVE_MEMORY_SANITIZER) &&                                     \
+    LIBC_HAS_BUILTIN(__builtin_constant_p)
+// Only perform MSAN unpoison in non-constexpr context.
 #include <sanitizer/msan_interface.h>
-#define MSAN_UNPOISON(addr, size) __msan_unpoison(addr, size)
+#define MSAN_UNPOISON(addr, size)                                              \
+  do {                                                                         \
+    if (!__builtin_constant_p(*addr)) {                                        \
+      __msan_unpoison(addr, size);                                             \
+    }                                                                          \
+  } while (0)
 #else
 #define MSAN_UNPOISON(ptr, size)
 #endif