[libc] Better IntegerToString API

This patch is an alternative to D155902. It provides the following benefits:
 - No buffer manual allocation and error handling for the general case
 - More flexible API : width specifier, sign and prefix handling
 - Simpler code

The more flexible API removes the need for manually tweaking the buffer afterwards, and so prevents relying on implementation details of IntegerToString.

Reviewed By: michaelrj, jhuber6

Differential Revision: https://reviews.llvm.org/D156981