[libc++][NFC] Use _LIBCPP_DEBUG_ASSERT in <string>

Use `_LIBCPP_DEBUG_ASSERT` instead of `_LIBCPP_ASSERT` and guarding it with `LIBCPP_DEBUG_LEVEL == 2`

Reviewed By: ldionne, #libc

Spies: libcxx-commits

Differential Revision: https://reviews.llvm.org/D115765