[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
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