[libc++] Introduce __make_uninitialized_buffer and use it instead of get_temporary_buffer

This will also be used in some PSTL backends.

Reviewed By: ldionne, #libc, Mordante

Spies: arichardson, mstorsjo, Mordante, sstefan1, jplehr, libcxx-commits

Differential Revision: https://reviews.llvm.org/D152208