This adds the main pieces of the last piece of printf, the converter.
This takes the completed format section from the parser and then
converts it to a string for the writer, which is why it was the last
piece to be written. So far it supports chars and strings, but more
pieces are coming. Additionally, it supports replacing all of the
conversion functions with user supplied versions at compile time to
allow for additional functionality.
Reviewed By: sivachandra
Differential Revision: https://reviews.llvm.org/D125327
6a22b185d6