[libc][msan] Fix "non-constexpr function '__msan_unpoison' cannot be used in a...

[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()`.