[libc] add scanf current position conversion

To add the current position (%n) conversion, some reorganization needed
to be done. The "write a number to this pointer using the length
modifier" utilities and a couple other shared parsing functions have
been moved into converter_utils.h. This made implementing
current_pos_converter very simple.

Reviewed By: lntue

Differential Revision: https://reviews.llvm.org/D142495