[libc++] Adds a make_string test helper function.

These function makes it easier to write generic unit tests for the
format header. It solves the issue where it's not possible to use
  `templated_prefix"foo"`
where `templated_prefix` resolves to: nothing, `L`, `u8`, `u`,
or `U`. The templated_prefix would be more faster during execution.

Reviewed By: ldionne, #libc, curdeius

Differential Revision: https://reviews.llvm.org/D93414