[libc][msan] Fix "non-constexpr function '__msan_unpoison' cannot be used in a constant expression" (#88719)

Prior to this patch, calling `cpp::bit_cast<T>` in `constexpr` expressions under `-fsanitize=memory` would fail with the following message "non-constexpr function '__msan_unpoison' cannot be used in a constant expression". This patch makes sure that the `__msan_unpoison` expression is guarded by `!__builtin_is_constant_evaluated()`.

Author: gchatelet

libc/src/__support/macros/sanitizer.h

@@ -47,14 +47,13 @@
 // Functions to unpoison memory
 //-----------------------------------------------------------------------------
 
-#if defined(LIBC_HAVE_MEMORY_SANITIZER) && __has_builtin(__builtin_constant_p)
+#if defined(LIBC_HAVE_MEMORY_SANITIZER)
 // Only perform MSAN unpoison in non-constexpr context.
 #include <sanitizer/msan_interface.h>
 #define MSAN_UNPOISON(addr, size) \
  do { \
- if (!__builtin_constant_p(*addr)) {  \
+ if (!__builtin_is_constant_evaluated()) \
  __msan_unpoison(addr, size); \
- } \
  } while (0)
 #else
 #define MSAN_UNPOISON(ptr, size)