[libc++] Simplify the string structures a bit more

This simplifies the string structs a bit more and the normal layout should not contain any undefined behaviour anymore. I don't think there is a way to achieve this in the alternate string mode without breaking the ABI.

Reviewed By: ldionne, #libc

Spies: libcxx-commits

Differential Revision: https://reviews.llvm.org/D125496