[libc++][ASan] Fix std::basic_string trait type (#91590)

Addresses the comment:
https://github.com/llvm/llvm-project/pull/79536#discussion_r1593652240

Changes the type to `void` instead of `false_type`.

The value is used here:


https://github.com/llvm/llvm-project/blob/6f1013a5b3f92d3ae6e378d6706584a2a44e6964/libcxx/include/__type_traits/is_trivially_relocatable.h#L35-L38